Incorrect line numbers in error messages when using distributed compilation from Windows

[mstange]

msvc.rs always passes "-EP" rather than "-E" in the preprocessing step:

sccache/src/compiler/msvc.rs

Line 689 in e66c9c1

cmd.arg("-EP")

"-EP" means "don't emit line number annotations".

Originally this was using "-E" (which does emit line number annotations), but it was changed to "-EP" in #723. This change mirrored a change to gcc.rs from #208. However, when support for distributed compilation was added in #249, gcc.rs was updated to only use "-P" when no distribution was happening:

4b04cb0#diff-33ed0b50ade3d44507b82055ded2e81fc58756670c39943d2b1305354c649dd6R353-R359

#249 did not make the equivalent change to msvc.rs.

msvc.rs should be updated similarly, so that it uses "-E" if may_dist and otherwise "-EP".