[Driver][SYCL] Add compile-time device library linking for SPIR-V targets (#196656)

This PR implements compile-time device library linking for SYCL
offloading to SPIR-V targets, using `libclang_rt.builtins.bc` - an
in-tree compiler-rt artifact produced alongside the existing
`libclang_rt.builtins.a` for `SPIRV64`.

## Motivation

SYCL device compilations targeting SPIR-V need access to compiler
builtins (integer arithmetic, floating-point helpers, etc.) at compile
time so the compiler can optimize across user code and builtins, inline
aggressively, and eliminate dead code. This PR lays the foundation by
wiring up the first in-tree device library : `libclang_rt.builtins.bc` -
using the same `-mlink-builtin-bitcode` mechanism already used by
`libclc` and `HIP`.

## Changes

**compiler-rt (compiler-rt/lib/builtins/CMakeLists.txt)**
For the `spirv64` target, after building
`libclang_rt.builtins-spirv64.a`, runs `llvm-link` to merge all bitcode
objects into a single `libclang_rt.builtins.bc`. This file is installed
to `<ResourceDir>/lib/spirv64-unknown-unknown/` alongside the static
archive. The `-mlink-builtin-bitcode` flag requires raw LLVM IR and
cannot unpack a static archive directly, hence the extra step.

**Clang Driver (clang/lib/Driver/ToolChains/SYCL.cpp)**
`SYCLToolChain::getDeviceLibs()` now constructs the path to
`libclang_rt.builtins.bc` under the clang resource directory and returns
it for `-mlink-builtin-bitcode` injection during `spirv64` device
compilation. If the file is absent (e.g. compiler-rt was not built with
spirv64 support), a specific diagnostic is emitted. Linking is
suppressed when `--no-offloadlib` is passed.

**Diagnostics (clang/include/clang/Basic/DiagnosticDriverKinds.td)**
Added `err_drv_no_compiler_rt_builtins_bc`, following the
`err_drv_libclc_not_found` community pattern, with actionable guidance
on how to build compiler-rt with SPIR-V support.

### Test Infrastructure

1. `clang/test/Driver/sycl-device-lib-spirv64.cpp` - new test verifying
-mlink-builtin-bitcode is passed for spirv64, suppressed with
--no-offloadlib, and that a missing .bc produces the correct diagnostic.
2. `clang/test/Driver/Inputs/spirv64-sycl/` - dummy resource-dir fixture
used by driver tests.
3. All existing SYCL driver tests updated to pass -`resource-dir
%S/Inputs/spirv64-sycl `on -fsycl RUN lines, making them independent of
the installed clang resource directory on CI runners.

## Behavior

### Default

```bash
clang -fsycl myprogram.cpp
# Device compilation includes:
# "-mlink-builtin-bitcode" "<ResourceDir>/lib/spirv64-unknown-unknown/libclang_rt.builtins.bc"
```

Automatically links device libraries from `<clang-install>/lib/` during
device compilation.

### Without device libraries
```bash
clang -fsycl --no-offloadlib myprogram.cpp
# -mlink-builtin-bitcode is suppressed
```
Missing .bc (compiler-rt not built with spirv64 support)
```bash
error: no compiler-rt builtins bitcode library '...' found in the clang resource directory;
build compiler-rt with SPIR-V support or pass '--no-offloadlib' to compile without it ```
GitOrigin-RevId: 5a650306e4e1069d194d8fcfeee821d7e424f9b3

lib/builtins/CMakeLists.txt

diff --git a/lib/builtins/CMakeLists.txt b/lib/builtins/CMakeLists.txt
index ea21c36..8761e25 100644
--- a/lib/builtins/CMakeLists.txt
+++ b/lib/builtins/CMakeLists.txt
@@ -1147,6 +1147,38 @@
       get_compiler_rt_output_dir(${arch} BUILTIN_LIB_OUTPUT_DIR)
       set_output_name(BUILTIN_LIB_OUTPUT_NAME clang_rt.builtins ${arch})
       file(WRITE "${BUILTIN_LIB_OUTPUT_DIR}/${BUILTIN_LIB_OUTPUT_NAME}.sources.txt" "${${arch}_SOURCES}")
+
+      # For spirv64, produce a combined bitcode file alongside the static archive.
+      # -mlink-builtin-bitcode expects raw LLVM IR and cannot unpack a static archive,
+      # so we use llvm-link to merge the archive members into a single .bc file.
+      if("${arch}" MATCHES "^spirv64")
+        find_program(LLVM_LINK llvm-link
+          PATHS ${LLVM_TOOLS_BINARY_DIR}
+          NO_DEFAULT_PATH)
+        if(LLVM_LINK)
+          set(SPIRV64_BC_OUTPUT "${BUILTIN_LIB_OUTPUT_DIR}/lib${BUILTIN_LIB_OUTPUT_NAME}.bc")
+          add_custom_command(
+            OUTPUT "${SPIRV64_BC_OUTPUT}"
+            COMMAND "${LLVM_LINK}"
+                    "$<TARGET_FILE:clang_rt.builtins-spirv64>"
+                    -o "${SPIRV64_BC_OUTPUT}"
+            DEPENDS clang_rt.builtins-${arch}
+            COMMENT "Linking lib${BUILTIN_LIB_OUTPUT_NAME}.bc"
+            VERBATIM)
+          add_custom_target(clang_rt.builtins-spirv64-bc ALL
+            DEPENDS "${SPIRV64_BC_OUTPUT}")
+          add_dependencies(builtins clang_rt.builtins-spirv64-bc)
+          get_compiler_rt_install_dir(${arch} BUILTIN_INSTALL_DIR)
+          install(FILES "${SPIRV64_BC_OUTPUT}"
+                  DESTINATION "${BUILTIN_INSTALL_DIR}"
+                  COMPONENT builtins)
+        else()
+          message(WARNING
+            "llvm-link not found (LLVM_TOOLS_BINARY_DIR='${LLVM_TOOLS_BINARY_DIR}'); "
+            "skipping libclang_rt.builtins.bc for spirv64")
+        endif()
+      endif()
+
       cmake_pop_check_state()
     endif ()
   endforeach ()