[clang][CodeGen] Ensure consistent `mustprogress` attribute emission

Emission of `mustprogress` attribute previously occurred only within
`EmitFunctionBody`, after generating the function body. Other routines
for function body creation may lack the attribute, potentially leading
to suboptimal optimizations later in the pipeline. Attribute emission
is now anticipated prior to generating the function body.

Fixes: https://github.com/llvm/llvm-project/issues/69833.