[clang][cmake] Add CLANG_DEFAULT_LINKER to ClangConfig.cmake.in (#158416)

Flang refers to this CMake variable since #149786, but this does not
work for standalone build.
This patch fixes it.

Author: parabola94

clang/cmake/modules/ClangConfig.cmake.in

@@ -10,6 +10,7 @@ set(CLANG_EXPORTED_TARGETS "@CLANG_EXPORTS@")
 set(CLANG_CMAKE_DIR "@CLANG_CONFIG_CMAKE_DIR@")
 set(CLANG_INCLUDE_DIRS "@CLANG_CONFIG_INCLUDE_DIRS@")
 set(CLANG_LINK_CLANG_DYLIB "@CLANG_LINK_CLANG_DYLIB@")
+set(CLANG_DEFAULT_LINKER "@CLANG_DEFAULT_LINKER@")
 
 # Provide all our library targets to users.
 @CLANG_CONFIG_INCLUDE_EXPORTS@