[clang-tblgen] Add -print-records and -dump-json modes.

Currently, if clang-tblgen is run without a mode option, it defaults
to the first mode in its 'enum Action', which happens to be
-gen-clang-attr-classes. I think it makes more sense for it to behave
the same way as llvm-tblgen, i.e. print a diagnostic dump if it's not
given any more specific instructions.

I've also added the same -dump-json that llvm-tblgen supports. This
means any tblgen command line (whether llvm- or clang-) can be
mechanically turned into one that processes the same input into JSON.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50771

llvm-svn: 340390