[CodeGen] Improve large stack frame diagnostic

Add statistics about how much memory is used, in variables, spills, and
unsafestack.

Issue #58168 describes some of the difficulty diagnosing stack size issues
identified by -Wframe-larger-than. D135488 addresses some of those issues by
giving developers a method to view the stack layout and thereby understand
where and how stack memory is used.

However, that solution requires an additional pass, when a short summary about
how the compiler has allocated stack memory can inform developers about where
they should investigate. When they need the complete context, D135488 can
provide them with a more comprehensive set of diagnostics.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D136484