[mlir] Initial version of C APIs

    Introduce an initial version of C API for MLIR core IR components: Value, Type,
    Attribute, Operation, Region, Block, Location. These APIs allow for both
    inspection and creation of the IR in the generic form and intended for wrapping
    in high-level library- and language-specific constructs. At this point, there
    is no stability guarantee provided for the API.

Reviewed By: stellaraccident, lattner

Differential Revision: https://reviews.llvm.org/D83310