[llvm-rc] Have the tokenizer discard single & block comments.

[llvm-rc] Have the tokenizer discard single & block comments.

This allows rc files to have comments.  Eventually we should
just use clang's c preprocessor, but that's a bit larger
effort for minimal gain, and this is straightforward.

Differential Revision: https://reviews.llvm.org/D38651

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315207 91177308-0d34-0410-b5e6-96231b3b80d8