[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