[IR] Add a Location to BlockArgument.

This adds the ability to specify a location when creating BlockArguments.
Notably Value::getLoc() will return this correctly, which makes diagnostics
more precise (e.g. the example in test-legalize-type-conversion.mlir).

This is currently optional to avoid breaking any existing code - if
absent, the BlockArgument defaults to using the location of its enclosing
operation (preserving existing behavior).

The bulk of this change is plumbing location tracking through the parser
and printer to make sure it can round trip (in -mlir-print-debuginfo
mode).  This is complete for generic operations, but requires manual
adoption for custom ops.

I added support for function-like ops to round trip their argument
locations - they print correctly, but when parsing the locations are
dropped on the floor.  I intend to fix this, but it will require more
invasive plumbing through "function_like_impl" stuff so I think it
best to split it out to its own patch.

Differential Revision: https://reviews.llvm.org/D102567