Add /bigobj compile option to MSVC build (microsoft#7228)

When targeting arm64 Debug, this error is detected:
`libclang\dxcrewriteunused.cpp(1,1): error C1128: number of sections
exceeded object file format limit: compile with /bigobj`
This PR adds a compile option for the folder that contains
dxcrewriteunused.cpp, so that the limit on the number of sections is
increased, and compilation may succeed.

Author: bob80905

tools/clang/tools/libclang/CMakeLists.txt

@@ -119,6 +119,7 @@ if(MSVC)
   # Each functions is exported as "dllexport" in include/clang-c.
   # KB835326
   set(LLVM_EXPORTED_SYMBOL_FILE)
+  add_compile_options(/bigobj)
 endif()
 
 # HLSL Change Starts