Add #include llvm-config.h to Locale.cpp which depends on LLVM_ON_WIN32.

Source code was assuming that llvm-config.h would be included somehow but
up to r247253 that added #include "llvm/Support/Compiler.h" to StringRef.h
the config file was not actually included. The inclusion of llvm-config.h
caused a change of behaviour in tools/clang/test/Frontend/source-col-map.c:
previously it would output the original UTF-8 but now it outputs <U+03B1>.

llvm-svn: 247409