Run misc-use-internal-linkage check over clang-tidy code.
Also fixed a couple of other clang-tidy warnings.
Apart from issues in header files, all '.cpp' in
`clang-tools-extra/clang-tidy` must be clang-tidy clear now.
These are identified by misc-include-cleaner. I've filtered out those
that break builds. Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.
