Don't pass wd4668 to clang-cl

[roblabla]

Fixes #1843

We should only pass /wd4668 when the compiler is MSVC, not when it's clang-cl, as the later does not understand that flag (and the issue it fixes is not relevant to clang-cl-based builds).

[quyykk]

@sylvestre sorry for the ping, but sccache is currently unusable with clang-cl without this fix so it would be great to get this PR reviewed and merged. thanks! 😄

[sylvestre]

Sorry I missed it.
Could you please add a test to make sure we don't regress?

[aeubanks]

the description is not quite right, see my comment in #1843 (comment)

but this approach is still right

we should also not pass -- for MSVC, but that's a separate bug

[glandium]

we should also not pass -- for MSVC, but that's a separate bug

I'd argue it's not sccache's business to remove command line arguments that were passed in.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7358e8c) 30.72% compared to head (0653933) 30.68%.

Files	Patch %	Lines
src/compiler/msvc.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1856      +/-   ##
==========================================
- Coverage   30.72%   30.68%   -0.05%     
==========================================
  Files          51       51              
  Lines       19190    19191       +1     
  Branches     9237     9238       +1     
==========================================
- Hits         5897     5888       -9     
+ Misses       7758     7757       -1     
- Partials     5535     5546      +11     

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