[mlir][spirv] NFC: expose builtin func op conversion pattern

This commit exposes the func op conversion pattern via a new
`populateBuiltinFuncToSPIRVPatterns` function from the standard
to SPIR-V conversion passs. This is structurally better given
that func op belongs to the builtin dialect. More importantly,
this makes the pattern reusable to other dialect to SPIR-V
dialect conversion as other dialect can well adopt builtin
func op instead of having its own. Besides, it's very common
to use func ops as test wrappers in lit tests, so test passes
will need to handle func ops too.

Differential Revision: https://reviews.llvm.org/D73421