[libclc] Add -fshort-wchar to windows build after f07988f

We were *hacking* the bc file in prepare-builtins by remvoing wchar_size
attributes since 8aa3513

Aftre prepare-builtin removal in f07988f,
we start to see linking failures in
    SYCL :: OptionalKernelFeatures/is_compatible/is_compatible_amdgcn.cpp
    SYCL :: OptionalKernelFeatures/is_compatible/is_compatible_nvptx64.cpp

This add the option to build libclc with -fshort-wchar on windows.
We are no using wchar anyhow.

Author: jsji

libclc/CMakeLists.txt

@@ -490,6 +490,10 @@ foreach( t ${LIBCLC_TARGETS_TO_BUILD} )
     # Add platform specific flags
     if(WIN32)
       list(APPEND build_flags -D_WIN32)
+      # wchar is not used in libclc,however,llvm ir contains wchar_size in attr
+      # the default on windows is 2 vs 4 in clang
+      # set it to 2 to avoid linking failures due to conflicts.
+      list(APPEND build_flags -fshort-wchar)
     elseif(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
       list(APPEND build_flags -D__APPLE__)
     elseif(${CMAKE_SYSTEM_NAME} MATCHES "Linux")