remove -Zi from MSVC compiler options

[Be-ing]

This is incompatible with compiler caches such as sccache and
clcache. If a project including Google Test specifies /Z7 instead,
building fails with:

sccache C:\PROGRA~2\MICROS~1\2019\ENTERP~1\VC\Tools\MSVC\1428~1.293\bin\Hostx64\x64\cl.exe  /nologo /TP -D__SSE2__ -D__SSE__ -I..\lib\googletest-1.10.x\googlemock\include -I..\lib\googletest-1.10.x\googlemock -I..\lib\googletest-1.10.x\googletest\include -I..\lib\googletest-1.10.x\googletest /DWIN32 /D_WINDOWS /W4 /GR  /MD /Z7 /O2 /Ob1 /DNDEBUG -GS -W4 -WX -wd4251 -wd4275 -nologo -J -Zi -D_UNICODE -DUNICODE -DWIN32 -D_WIN32 -DSTRICT -DWIN32_LEAN_AND_MEAN -wd4702 -DGTEST_HAS_PTHREAD=0 -EHsc -D_HAS_EXCEPTIONS=1  /Gy /showIncludes /Folib\googletest-1.10.x\googlemock\CMakeFiles\gmock_main.dir\src\gmock-all.cc.obj /Fdbin\gmock_main.pdb /FS -c ..\lib\googletest-1.10.x\googlemock\src\gmock-all.cc
FAILED: lib/googletest-1.10.x/googlemock/CMakeFiles/gmock_main.dir/src/gmock-all.cc.obj
..\lib\googletest-1.10.x\googletest\src\gtest-all.cc: fatal error C1041: cannot open program database 'D:\a\mixxx\mixxx\build\bin\gmock_main.pdb'; if multiple CL.EXE write to the same .PDB file, please use /FS
cl : Command line warning D9025 : overriding '/Z7' with '/Zi'

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[Be-ing]

@googlebot I signed it!

[pmisik]

For most Windows people it may look weird why not use default -Zi and use -Z7 rather.
-Zi cause that symbols - pdb are generated during link stage by mspdbsrv.exe service started on demand.
If you compile single project it is worth optimization Microsoft done.
When you are CI guy building hunderts projects in parallel you'll face very frequent crashes of mspdbsrv due to out of memory (especially in cases using Default VCToolsets -settings).
-Z7 cause compiler generates symbols data into .obj (objs are bigger) and linker just merge them. In optimal cases (you don't need link masm [not having support for /Z7] you should be able avoid interaction of mspdbsrv).
This will improve builds reliability.

Disclaimer:
I don't use cmake. So I'm not sure change is ok.
I work in team use msbuild intensively.
I just added comment why request is really worth to consider.
If you build in parallel on NUMA machines -Z7 may improve throughput as it has opportunity eliminate FSB saturation.

[Be-ing]

I don't think a project designed to be built into other projects should be setting any choice of how the debugging info is handled.

[Be-ing]

For me the pros and cons of /Zi versus /Z7 are pretty much irrelevant. /Zi doesn't work with compiler caches, therefore I can't use it.

[Be-ing]

Thank you for merging.