libclang: Update comment about USEDLIBS in c-*-test/Makefile.

"Note that 'USEDLIBS' must include all of the core clang libraries when -static is given to linker on cygming."

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173124 91177308-0d34-0410-b5e6-96231b3b80d8