[update_cc_test_checks.py] Use -ast-dump=json to get mangled name

Summary:
Using c-index-test is fragile since it does not parse all the clang
arguments that are used in the RUN: line. This can result in incorrect
mangled names that do not match any of the generated IR.
For example macOS triples include a leading underscore (which was handled
with a hack in the current script). For the CHERI target we have added
new qualifiers which affect C++ name mangling, but will be included added
by update_cc_test_checks since it parses the source file with the host
triple because it ignores the -triple= argument passed to clang -cc1.

Using the new feature of including the mangled name in the JSON AST dump
(see D69564), we can parse the output of the RUN: command with
"-fsyntax-only -ast-dump=json" appended.
This should make the script less fragile and also forks one process less.

Reviewers: MaskRay, xbolva00

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69565