[mlir] Optimize the parsing of ElementsAttr hex strings

This revision optimizes the parsing of hex strings by using the checked variant of llvm::fromHex, and adding a specialized method to Token for extracting hex strings. This leads a large decrease in compile time when parsing large hex constants (one example: 2.6 seconds -> 370 miliseconds)

Differential Revision: https://reviews.llvm.org/D90266