[llvm] [LowerTypeTests] Skip declarations when determining Thumb support (PR #129599)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> If the target triple reliably contained enough information to distinguish Armv6-M from later versions, I agree!

clang generally rewrites the triple in a way that makes querying the subarch from the triple reliable.  Not sure if we expect non-clang frontends to do the same thing.

https://github.com/llvm/llvm-project/pull/129599