[clang][codegen] Remember string used to create llvm::Regex for optimization remarks

Regular expression patterns passed through the command line are being used to create an instances of `llvm::Regex` and thrown away.

There is no API to serialize `Regex` back to the original pattern. This means we have no way to reconstruct the original pattern from command line. This is necessary for serializing `CompilerInvocation`.

This patch stores the original pattern string in `CodeGenOptions` alongside the `llvm::Regex` instance.

Reviewed By: dexonsmith, thegameg

Differential Revision: https://reviews.llvm.org/D96036