[CIR] Upstream a basic version of class LexicalScope (#131945)

Upstream the parts of class `CIRGenFunction::LexicalScope` that
implement function return values. There is a bit of other functionality
in here, such as the implicit `cir.yield` at the end of a non-function
scope, but this is mostly about function returns.

The parts of `LexicalScope` that handle calling destructors, switch
statements, ternary expressions, and exception handling still need to be
upstreamed.

There is a change in the generated ClangIR (which is why many of the
tests needed updating). Return values are stored in the
compiler-generated variable `__retval` rather than being passed to the
`cir.return` op directly. But there should be no change in the behavior
of the generated code.

0 files changed, 0 insertions, 0 deletions