[IR] Add an ImplicitLocOpBuilder helper class for building IR with the same loc.

One common situation is to create a lot of IR at a well known location,
e.g. when doing a big rewrite from one dialect to another where you're expanding
ops out into lots of other ops.

For these sorts of situations, it is annoying to pass the location into
every create call.  As we discused in a few threads on the forum, a way to help
with this is to produce a new sort of builder that holds a location and provides
it to each of the create<> calls automatically.

This patch implements an ImplicitLocOpBuilder class that does this.  We've had
good experience with this in the CIRCT project, and it makes sense to upstream to
MLIR.

I picked a random pass to adopt it to show the impact, but I don't think there is
any particular need to force adopt it in the codebase.

Differential Revision: https://reviews.llvm.org/D93717