[CIR] Upstream support for thread local variables

[andykaylor]

Overview

Upstream support for handling thread local variables in CIR.

This is a somewhat large task that will require modifications to many existing pieces of code. The places where additional handling is needed are mostly marked by either cir::MissingFeatures::opGlobalThreadLocal(), cir::MissingFeatures::opLoadStoreThreadLocal(), or cir::MissingFeatures::opAllocaTLS() assertions.

Suggested minimal test case

Existing incubator tests

clang/test/CIR/CodeGen/tls.c

Classic codegen tests that should be considered

clang/test/CodeGen/tls-model.cpp
clang/test/CodeGen/thread-specifier.c

[andykaylor]

I forgot I had already filed this as #153270