[clang-cl] Implement /X

/X makes cl stop looking in %INCLUDE%. Implement this for clang-cl.

As it turns out, the return in ToolChains/MSVC.cpp, AddClangSystemIncludeArgs()
for -nostdlibinc is already in the right place (but -nostdlibinc isn't exposed
by clang-cl), so just alias /X to that.

https://reviews.llvm.org/D43888

llvm-svn: 326357