[mlir][CAPI] Expose the rest of MLIRContext's constructors

It's recommended practice that people calling MLIR in a loop
pre-create a LLVM ThreadPool and a dialect registry and then
explicitly pass those into a MLIRContext for each compilation.
However, the C API does not expose the functions needed to follow this
recommendation from a project that isn't calling MLIR's C++ dilectly.

Add the necessary APIs to mlir-c, including a wrapper around LLVM's
ThreadPool struct (so as to avoid having to amend or re-export parts
of the LLVM API).

Reviewed By: makslevental

Differential Revision: https://reviews.llvm.org/D153593