Fix the build of the include-fixer plugin for some shared cmake builds and MSVC.

- The include fixer plugin does not directly depend on pthread, but can
pick up pthread references transitively through inlining. Just add
pthreads to the linked libs unconditionally.
- MSVC emits bogus warnings when including <future> and building without
exceptions. Blacklist the warnings explicitly.

llvm-svn: 291892