[llvm-rc] Look for "clang-<major>" when locating a suitable preprocessor

In some cases, there's no adjacent executable named "clang" or
"clang-cl", but one name "clang-<major>". This logic doesn't
cover every possible deployment setup of course, but should
cover more fairly common/reasonable cases.

See
https://github.com/curl/curl-for-win/commit/caaae171ac43af5b883403714dafd42030d8de61#commitcomment-105808524
for discussion about a case where this would have been helpful.

Differential Revision: https://reviews.llvm.org/D146794