[clang-tidy] Add "clang-tidy as a clang plugin" skeleton.

[clang-tidy] Add "clang-tidy as a clang plugin" skeleton.

This doesn't really do much at the moment. You can load it via libclang
and set the -checks via an extra command line argument as illustrated in
the test case. Support for other options (including headers check) is
currently missing. Also when using this with libclang some checks may
not work with the precompiled preamble in place.

This can be used to easily show clang-tidy warnings in an editor
integration as all that's needed is adding command line flags that are
passed into libclang. Warnings and FixIts are exposed via the existing
CXDiagnostic machinery.

Differential Revision: http://reviews.llvm.org/D17807

llvm-svn: 262595