[cmake] Pass -resource-dir to external projects as CMAKE_<LANG>_FLAGS_INIT instead of as a force override

It's useful to use toolchain files for configuring runtimes builds.
Setting CMAKE_<LANG>_FLAGS is however forceful and precludes initialization from the _INIT variant meant to be set by toolchains.
The toolchain file still needs to append instead of just setting and be idempotent (`include_guard()`), but this way at least a special toolchain to account for LLVM's won't be required.

Author: tambry

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

@@ -287,7 +287,7 @@ function(llvm_ExternalProject_Add name source_dir)
       endif()
       set(flag_types ASM C CXX MODULE_LINKER SHARED_LINKER EXE_LINKER)
       foreach(type ${flag_types})
-        set(${type}_flag -DCMAKE_${type}_FLAGS=-resource-dir=${resource_dir})
+        set(${type}_flag -DCMAKE_${type}_FLAGS_INIT=-resource-dir=${resource_dir})
       endforeach()
       string(REPLACE ";" "|" flag_string "${flag_types}")
       foreach(arg ${ARG_CMAKE_ARGS})