Fix a failing assertion with implicit function definitions

Fix a failing assertion with implicit function definitions

When implicitly defining a function in C, we would try to find an
appropriate declaration context for the function to be declared within.
However, we did not account for GNU statement expressions, which
masquerade as a compound statement and can be used in other contexts
such as within structure member declarations.

Fixes https://github.com/llvm/llvm-project/issues/48579