[LowerTypeTests] Skip declarations when determining Thumb support
When looping over all functions in a module to determine whether any of
them is built with support for B.W, we can skip declarations since those
do not have an associated target-feature attribute.
This was found by the assertion from https://github.com/llvm/llvm-project/pull/129600
Reviewed By: statham-arm
Pull Request: https://github.com/llvm/llvm-project/pull/129599