Strip preceeding -Xclang when stripping -fcolor-diagnostics or -fdiagnostics-color

Summary: Fixes https://github.com/clangd/clangd/issues/279. We were removing the color options but not the preceeding -Xclang which causes errors since the -Xclang would now apply to the next option in the list of options. Now, when removing a color option, we check if there was a preceeding -Xclang and remove it as well.

Patch By @DaanDeMeyer !

Reviewers: sammccall, kadircet

Reviewed By: sammccall

Subscribers: ilya-biryukov, usaxena95

Differential Revision: https://reviews.llvm.org/D75019

(cherry picked from commit da236f23)