[CMake] Use write_basic_package_version_file for LLVM

Use the CMake 3.13 features of CMakeConfigPackageHelpers to generate
LLVMConfigVersion.cmake with proper architecture detection, major+minor
version matching, etc.

Differential Revision: https://reviews.llvm.org/D99451

Author: alexreinking

llvm/cmake/modules/CMakeLists.txt

@@ -127,11 +127,13 @@ configure_file(
   ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/LLVMConfig.cmake
   @ONLY)
 
-# Generate LLVMConfigVersion.cmake for build and install tree.
-configure_file(
-  LLVMConfigVersion.cmake.in
-  ${llvm_cmake_builddir}/LLVMConfigVersion.cmake
-  @ONLY)
+# Generate LLVMConfigVersion.cmake for build tree (later copied to install tree).
+include(CMakePackageConfigHelpers)
+write_basic_package_version_file(
+  "${llvm_cmake_builddir}/LLVMConfigVersion.cmake"
+  VERSION "${PACKAGE_VERSION}"
+  COMPATIBILITY SameMinorVersion
+)
 
 if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   get_property(llvm_has_exports GLOBAL PROPERTY LLVM_HAS_EXPORTS)