Remove X86Subtarget from the X86FrameLowering constructor since
we can just pass in the values we already know and we're not
caching the subtarget anymore.
Implement -Wframe-larger-than backend diagnostic

Add driver and frontend support for the GCC -Wframe-larger-than=bytes warning.
This is the first GCC-compatible backend diagnostic built around LLVM's
reporting feature.

This commit adds infrastructure to perform reverse lookup from mangled names
emitted after LLVM IR generation. We use that to resolve precise locations and
originating AST functions, lambdas or block declarations to produce seamless
codegen-guided diagnostics.

An associated change, StringMap now maintains unique mangled name strings
instead of allocating copies. This is a net memory saving in C++ and a small
hit for C where we no longer reuse IdentifierInfo storage, pending further
optimisation.
Provide fallback locations for backend remarks

Instead of disembodied diagnostics when debug info is disabled it's now
possible to identify the associated function's location in order to provide
some amount of of context.

We use the definition's body right brace location to differentiate the fallback
from diagnostics that genuinely relate to the function declaration itself (a
convention also used by gcc).