[cmake] Allow to set C/C++ linker (#4462)

Backport an LLVM cmake option to specify the C/C++ linker to use.
This implementation is simplified and unlike the llvm-project one
does not contain any linker-specific tweaks.

On my system, setting `-DLLVM_USE_LINKER=mold` reduces the full build
time from 81s to 43s.

cmake/modules/HandleLLVMOptions.cmake

@@ -173,6 +173,18 @@ function(add_flag_or_print_warning flag name)
   endif()
 endfunction()
 
+if( LLVM_USE_LINKER )
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fuse-ld=${LLVM_USE_LINKER}")
+  check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
+  if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
+    message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
+  endif()
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+  append("-fuse-ld=${LLVM_USE_LINKER}"
+    CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+endif()
+
 if( LLVM_ENABLE_PIC )
   if( XCODE )
     # Xcode has -mdynamic-no-pic on by default, which overrides -fPIC. I don't