[llvm-rc] Run clang to preprocess input files

Allow opting out from preprocessing with a command line argument.

Update tests to pass -no-preprocess to make it not try to use clang
(which isn't a build level dependency of llvm-rc), but add a test that
does preprocessing under clang/test/Preprocessor.

Update a few options to allow them both joined (as -DFOO) and separate
(-D BR), as rc.exe allows both forms of them.

With the verbose flag set, this prints the preprocessing command
used (which differs from what rc.exe does).

Tests under llvm/test/tools/llvm-rc only test constructing the
preprocessor commands, while tests under clang/test/Preprocessor test
actually running the preprocessor.

Differential Revision: https://reviews.llvm.org/D100755