[update_cc_test_checks.py] Handle extern "C" and namespaces

Summary:
My change to use the clang AST JSON dump did not handle functions declared
inside scopes other than the root TranslationUnitDecl. After this change
update_cc_test_checks.py also works for C++ test cases that use extern "C"
and namespaces.

Reviewers: MaskRay

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70389