[mlir] Add `OpAsmPrinter::printOptionalLocationSpecifier`

This is the corresponding method to
`OpAsmParser::parseOptionalLocationSpecifier` that prints a location
`loc(...)` based on the op printing flags. Together, these two functions
allow propagating user-level location info outside of their usual spots.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D134910