[llvm][StringExtras] Merge StringExtras from MLIR into LLVM

Summary:
This revision adds two utilities currently present in MLIR to LLVM StringExtras:

* convertToSnakeFromCamelCase
Convert a string from a camel case naming scheme, to a snake case scheme

* convertToCamelFromSnakeCase
Convert a string from a snake case naming scheme, to a camel case scheme

Differential Revision: https://reviews.llvm.org/D78167