Include the mangled name in -ast-dump=json

I am planning to use this feature to make update_cc_test_checks.py less fragile
by obtaining the mangled names directly from -ast-dump=json. Currently,
it uses c-index-test which ignores the -triple=, etc. arguments that are
in the RUN: line and therefore does not generate checks for some targets.

The AST dump tests were updated using the following command:
`python $LLVM_BINDIR/gen_ast_dump_json_test.py --update --source $LLVM_SRC/clang/test/AST/*-json.*`

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: rsmith, MaskRay, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69564