[UnsafeBufferUsage] Check for uninstantiated default arguments to prevent crash. (#188817)
Fix a crash introduced by
https://github.com/llvm/llvm-project/pull/184899
The -Wunsafe-buffer-usage analysis was crashing when it encountered a
template function with a default argument that hadn't been instantiated
yet. This occurred in populateStmtsForFindingGadgets when it attempted
to access the default argument of each parameter.
This fix adds a check to ensure the default argument is instantiated
before attempting to access it.
Assisted-by: Gemini