[CIR] Upstream support for C++ name mangling

[andykaylor]

Overview

Upstream support for C++ name mangling. Currently, we're still emitting unmangled names for C++ functions. As we start to implement C++ features, we're developing a large backlog of tests that will need to be updated when name mangling is enabled.

This will require introducing the infrastructure for target-specific CXXABI handling.

Suggested minimal test case

void fn() {}

Existing incubator tests

This is thoroughly covered by all C++ tests, but we are missing mangling-specific tests. There are many such tests in clang/test/CodeGenCXX/mangle*.cpp