[MLIR] Expose LocationAttrs in the C API

This patch adds three functions to the C API:
- mlirAttributeIsALocation: returns true if the attribute is a LocationAttr,
  false otherwise.
- mlirLocationGetAttribute: returns the underlying LocationAttr of a Location.
- mlirLocationFromAttribute: gets a Location from a LocationAttr.

Reviewed By: mikeurbach, Mogball

Differential Revision: https://reviews.llvm.org/D142182