[llvm] [StandardInstrumentation] Annotate loops with the function name (PR #90756)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

makes sense to me, although bikeshedding a bit, I'd prefer
`Running pass: SimpleLoopUnswitchPass on loop %loop_begin in function test6`

happy to hear any other tweaks to what actually gets printed

https://github.com/llvm/llvm-project/pull/90756