Reland "[CMake] Bumps minimum version to 3.20.0.

This reverts commit d763c6e5e2d0a6b34097aa7dabca31e9aff9b0b6.

Adds the patch by @hans from
https://github.com/llvm/llvm-project/issues/62719
This patch fixes the Windows build.

d763c6e5e2d0a6b34097aa7dabca31e9aff9b0b6 reverted the reviews

D144509 [CMake] Bumps minimum version to 3.20.0.

This partly undoes D137724.

This change has been discussed on discourse
https://discourse.llvm.org/t/rfc-upgrading-llvms-minimum-required-cmake-version/66193

Note this does not remove work-arounds for older CMake versions, that
will be done in followup patches.

D150532 [OpenMP] Compile assembly files as ASM, not C

Since CMake 3.20, CMake explicitly passes "-x c" (or equivalent)
when compiling a file which has been set as having the language
C. This behaviour change only takes place if "cmake_minimum_required"
is set to 3.20 or newer, or if the policy CMP0119 is set to new.

Attempting to compile assembly files with "-x c" fails, however
this is workarounded in many cases, as OpenMP overrides this with
"-x assembler-with-cpp", however this is only added for non-Windows
targets.

Thus, after increasing cmake_minimum_required to 3.20, this breaks
compiling the GNU assembly for Windows targets; the GNU assembly is
used for ARM and AArch64 Windows targets when building with Clang.
This patch unbreaks that.

D150688 [cmake] Set CMP0091 to fix Windows builds after the cmake_minimum_required bump

The build uses other mechanism to select the runtime.

Fixes #62719

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D151344

GitOrigin-RevId: cbaa3597aaf6273e66b3f445ed36a6458143fe6a
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c5a7b24..6489aa1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,24 +3,19 @@
 # An important constraint of the build is that it only produces libraries
 # based on the ability of the host toolchain to target various platforms.
 
-cmake_minimum_required(VERSION 3.13.4)
+cmake_minimum_required(VERSION 3.20.0)
+
+set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
+include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
+  NO_POLICY_SCOPE)
 
 # Check if compiler-rt is built as a standalone project.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR COMPILER_RT_STANDALONE_BUILD)
   project(CompilerRT C CXX ASM)
   set(COMPILER_RT_STANDALONE_BUILD TRUE)
   set_property(GLOBAL PROPERTY USE_FOLDERS ON)
-  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
-    message(WARNING
-      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
-      "minimum version of CMake required to build LLVM will become 3.20.0, and "
-      "using an older CMake will become an error. Please upgrade your CMake to "
-      "at least 3.20.0 now to avoid issues in the future!")
-  endif()
 endif()
 
-set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
-
 # Add path for custom compiler-rt modules.
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
diff --git a/lib/builtins/CMakeLists.txt b/lib/builtins/CMakeLists.txt
index a302306..c3e22a8 100644
--- a/lib/builtins/CMakeLists.txt
+++ b/lib/builtins/CMakeLists.txt
@@ -3,14 +3,7 @@
 # architecture-specific code in various subdirectories.
 
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
-  cmake_minimum_required(VERSION 3.13.4)
-  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
-    message(WARNING
-      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
-      "minimum version of CMake required to build LLVM will become 3.20.0, and "
-      "using an older CMake will become an error. Please upgrade your CMake to "
-      "at least 3.20.0 now to avoid issues in the future!")
-  endif()
+  cmake_minimum_required(VERSION 3.20.0)
 
   set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
   project(CompilerRTBuiltins C ASM)
diff --git a/lib/crt/CMakeLists.txt b/lib/crt/CMakeLists.txt
index 771652f..32fd61b 100644
--- a/lib/crt/CMakeLists.txt
+++ b/lib/crt/CMakeLists.txt
@@ -1,12 +1,5 @@
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
-  cmake_minimum_required(VERSION 3.13.4)
-  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
-    message(WARNING
-      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
-      "minimum version of CMake required to build LLVM will become 3.20.0, and "
-      "using an older CMake will become an error. Please upgrade your CMake to "
-      "at least 3.20.0 now to avoid issues in the future!")
-  endif()
+  cmake_minimum_required(VERSION 3.20.0)
 
   set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
   project(CompilerRTCRT C)