Fix a cmake error when using the Xcode generator. (#119403)

I’m seeing a series of errors when trying to run the cmake configure
step on macOS when the cmake generator is set to Xcode. All is well if I
use the Ninja or Unix Makefile generators. Messages are all of the form:
~~~
CMake Error at …llvm-project/clang/cmake/modules/AddClang.cmake:120
(target_compile_definitions):
  Cannot specify compile definitions for target "obj.clangBasic" which
  is not built by this project.
Call Stack (most recent call first):
  …llvm-project/clang/lib/Basic/CMakeLists.txt:57 (add_clang_library)
~~~
The remaining errors are similar but mention targets obj.clangAPINotes,
obj.clangLex, obj.clangParse, and so on.

The regression appears to have been introduced by commit 09fa2f012fcc
(Oct 14 2024) which added the code in this area.

My proposed solution is simply to add a test to ensure that the obj.x
target exists before setting its compile definitions. There is precedent
doing just this in both clang/cmake/modules/AddClang.cmake and
clang/lib/support/CMakeLists.txt as well as in the “MSVC AND NOT
CLANG_LINK_CLANG_DYLIB” path immediately above the offending line.

I’ve also made a couple of grammatical tweaks in the comments
surrounding this code.

In case it's relevant, the cmake settings and definitions I've used to
trigger these errors is:
~~~bash
GENERATOR="Xcode"
OUTDIR=build_macos
cmake \
-S "$SCRIPT_DIR/llvm" \
-B "$SCRIPT_DIR/$OUTDIR" \
-G "$GENERATOR" \
-D CMAKE_BUILD_TYPE=Release \
-D CMAKE_OSX_ARCHITECTURES=arm64 \
-D LLVM_PARALLEL_LINK_JOBS=1 \
-D LLVM_ENABLE_PROJECTS="clang;lld" \
-D LLVM_TARGETS_TO_BUILD=RISCV \
-D LLVM_DEFAULT_TARGET_TRIPLE=riscv32-unknown-elf \
-D LLVM_OPTIMIZED_TABLEGEN=Yes
~~~
(cmake v3.31.1, Xcode 16.1. I know that not all of these variables are
useful for the Xcode generator!)

Co-authored-by: Paul Bowen-Huggett <phuggett@keysom.io>