[XRay][clang] Make -fxray-modes= additive

Summary:
This allows us to do the following:

  clang -fxray-modes=none ... -fxray-modes=xray-basic

It's important to be able to do this in cases where we'd like to
specialise the configuration for the invocation of the compiler, in
various scripting environments.

This is related to llvm.org/PR37066, a follow-up to D45474.

Reviewers: eizan, kpw, pelikan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45610

llvm-svn: 329989