[CMake][Modules] libclang: Ignore _CINDEX_LIB_ and CLANG_TOOL_EXTRA_BUILD for -fmodules.

CLANG_TOOL_EXTRA_BUILD doesn't affect headers.

_CINDEX_LIB_ is defined when the target is SHARED.
On Win32, it affects clang-c/Platform.h and it shouldn't be ignored.

This is part of https://reviews.llvm.org/D35559

llvm-svn: 309557