--deduplicate unions /Fp (PCH output path) across merged entries, producing invalid multi- /Fp command lines

[kuchungmsft]

When  --deduplicate  collapses the multiple compile entries for a source file that is built in more than one project,  MergeEntries  treats the PCH switches ( /Fp ,  /Yu ,  /Yc ) as generic "other" flags and unions them. Because each project emits a distinct  /Fp , the merged entry ends up carrying multiple conflicting  /Fp  switches.  /Fp  is a single-valued, last-wins cl.exe switch (it names the PCH file), so the resulting command line cannot be honored coherently by a compiler or an IntelliSense engine.

Repro

1. A repo where source files are compiled in multiple projects with PCH (e.g. Windows Terminal  OpenConsole.slnx ).
2. Run the extractor with dedup:
   dotnet run --project msbuild-extractor-sample.csproj -c Release --
   --solution \OpenConsole.slnx -c Debug -a x64
   --deduplicate --prefer-configuration Debug --prefer-platform x64
   --msbuild-path <...> --vc-tools-install-dir <...> --cl-path <...>
   -o .mscppls\compile_commands.json
3. Inspect the output for entries with more than one  /Fp .

Expected

Each deduplicated entry has at most one PCH identity: a single  /Fp  and its matching  /Yu  or  /Yc , taken from the preferred (config/platform-winning) entry, consistent with how  IsPreferred  already selects the base command.

Actual

Merged entries carry multiple distinct  /Fp  paths. In a Windows Terminal Debug/x64 extraction (extractor  msbuild-extractor-sample/1.0.0 ): 562 deduplicated entries, 53 entries with more than one  /Fp , max 5 on a single entry. Worst case is a PCH-create source:

precomp.cpp (/Yc) 5 x /Fp:
/Fp...\Host.EXE\OpenConsole.pch
/Fp...\Host.FuzzWrapper\OpenConsoleFuzzer.pch
/Fp...\Host\ConhostV2Lib.pch
/Fp...\Host.Tests.Unit\Conhost.Unit.Tests.pch
/Fp...\Host.unittest\ConhostV2Lib.unittest.pch