Add tests for clang plugins

Somehow the entire plugin infrastructure went wholly untested until now.

The only plugins available for use in testing are the examples, so plugin tests
will only be run if CLANG_BUILD_EXAMPLES is enabled in the build.

(The examples should really be enabled by default, not just to aid testing but
also to prevent bitrot in some key user-facing code. I'll propose that
shortly.)

Requires supporting changes in LLVM r198746.