Support rewrite_includes_only with clang-cl

[mstange]

To avoid large swathes of compiler warnings during distributed compilation, we try to limit client-side preprocessing to only resolve includes, and not substitute #defines etc. This mode is enabled when rewrite_includes_only = true is set in the dist config.

However, this mode is currently unimplemented on Windows: msvc.rs ignores the rewrite_includes_only parameter.

sccache/src/compiler/msvc.rs

Line 69 in e66c9c1

_rewrite_includes_only: bool,

MSVC's cl.exe does not appear to support an argument to unlock this mode. However, clang-cl does, in an underhanded way, with the -clang:-frewrite-includes argument. So we can support rewrite_includes_only in msvc.rs if clang-cl is used.

With this change, I've been able to build Firefox on Windows with distributed sccache successfully. Without this change, the build was 1. failing because some directories had -Werror enabled and 2. extremely slow, because the console delayed everything by trying to keep up with the huge amounts of warning output.