[mlir] expose standard types to C API

Provide C API for MLIR standard types. Since standard types live under lib/IR
in core MLIR, place the C APIs in the IR library as well (standard ops will go
into a separate library). This also defines a placeholder for affine maps that
are necessary to construct a memref, but are not yet exposed to the C API.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D86094