[llvm] Construct option's prefixed name at compile-time

Some Clang command-line handling code could benefit from the option's prefixed name being a `StringLiteral`. This patch changes the `llvm::opt` TableGen backend to generate and emit that into the .inc file.

Depends on D157028.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D157029