-Xclang -fno-pch-timestamp disables caching with clang

[craigscott-crascit]

When PCH generation is enabled with clang, the -Xclang -fno-pch-timestamp flags most likely should be used (for background, see the ccache docs which mention this). Projects that are supporting both ccache and sccache, or that are switching from ccache to sccache, may already have -Xclang -fno-pch-timestamp being added. Since it may legitimately be present, sccache should ideally handle that, but right now it rejects it with the dreaded "Can't handle UnknownFlag arguments with -Xclang" error.

This appears to be a very similar case to #1266. I suspect a fix very similar in nature to #1547 can be applied here as well for -fno-pch-timestamp.