plugins: make swiftCompilerModules optional

[compnerd]

This is not always available, and is definitely not yet available on
Windows due to a completely broken build system. This optionalizes the
dependency to repair the Windows build.

[compnerd]

@swift-ci please test

[compnerd]

CC: @eeckstein @JDevlieghere

[eeckstein]

Why is this needed?
If bootstrapping=off, then swiftCompilerModules is aliased to swiftCompilerStub. So it should always be compilable, even if no swift modules are linked to the tool.

[compnerd]

Interesting, well, something is definitely not working as expected on the Swift side as I'm seeing a bunch of failures in CI: https://ci-external.swift.org/job/swift-PR-windows/19527/consoleText

FAILED: bin/lldb-server.exe 
cmd.exe /C "cd . && "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" -E vs_link_exe --intdir=tools\lldb-server\CMakeFiles\lldb-server.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100177~1.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\100177~1.0\x64\mt.exe --manifests  -- C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\MSVC\1429~1.300\bin\Hostx64\x64\link.exe /nologo @CMakeFiles\lldb-server.rsp  /out:bin\lldb-server.exe /implib:lib\lldb-server.lib /pdb:bin\lldb-server.pdb /version:0.0 /INCREMENTAL:NO /STACK:10000000 /INCREMENTAL:NO /subsystem:console  && cd ."
LINK: command "C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\MSVC\1429~1.300\bin\Hostx64\x64\link.exe /nologo @CMakeFiles\lldb-server.rsp /out:bin\lldb-server.exe /implib:lib\lldb-server.lib /pdb:bin\lldb-server.pdb /version:0.0 /INCREMENTAL:NO /STACK:10000000 /INCREMENTAL:NO /subsystem:console /MANIFEST /MANIFESTFILE:bin\lldb-server.exe.manifest" failed (exit code 1181) with the following output:
LINK : fatal error LNK1181: cannot open input file 'swiftCompilerModules.lib'

[eeckstein]

ok, I'm fine with this change. Can you please add a comment that this is only a workaround for this problem on windows?

[eeckstein]

Actually, it will fail without linking the stub, because it will need the initializeSwiftModules symbol

[compnerd]

Seems that there is a dependency issue between the two that causes this. How do you expect the dependency to flow for this?

[eeckstein]

This is a bug: the initializeSwiftModules stub library is not added to the exported targets. The fix is here: swiftlang/swift#40712

[compnerd]

Superseded by #3737.