Fix build errors for SourceKit on Linux

[alblue]

The sourcekitd-repl, sourcekitd-test and complete-test use symbols
from both the coverage and lto modules, specifically
llvm::coverage::CoverageFilenamesSectionWriter::write and
llvm::lto::thinBackend. Adding these to the list of dependencies
allows SourceKit to be built on Linux.

Resolves SR-3223.

[alblue]

I have fixed this to also allow the unit tests to run successfully.

[alblue]

Can someone trigger a smoke test for this? The checks appear to have been hanging for some time.

[modocache]

@swift-ci Please smoke test

[alblue]

Thanks!

[jrose-apple]

@nkcsgexi or @akyrtzi, sanity check?

[akyrtzi]

Could you try removing coverage from tools/SourceKit/lib/SwiftLang/CMakeLists.txt ?
A bunch of such dependencies were added a while back to workaround linker errors on linux but they may be unnecessary now. Also stuff like mc, mcparser, etc.

It would be great if you could check if we can reduce dependencies now.

[alblue]

No, it doesn't compile if you remove coverage from tools/SourceKit/lib/SwiftLang/CMakeLists.txt

.../swift-build/build/Ninja-ReleaseAssert/llvm-linux-x86_64/./lib/libclangCodeGen.a(CoverageMappingGen.cpp.o):.../swift-build/llvm/tools/clang/lib/CodeGen/CoverageMappingGen.cpp:function clang::CodeGen::CoverageMappingModuleGen::emit(): error: undefined reference to 'llvm::coverage::CoverageFilenamesSectionWriter::write(llvm::raw_ostream&)'
.../swift-build/build/Ninja-ReleaseAssert/llvm-linux-x86_64/./lib/libclangCodeGen.a(CoverageMappingGen.cpp.o):.../swift-build/llvm/tools/clang/lib/CodeGen/CoverageMappingGen.cpp:function clang::CodeGen::CoverageMappingGen::emitCounterMapping(clang::Decl const*, llvm::raw_ostream&): error: undefined reference to 'llvm::coverage::CoverageMappingWriter::write(llvm::raw_ostream&)'
.../swift-build/build/Ninja-ReleaseAssert/llvm-linux-x86_64/./lib/libclangCodeGen.a(CoverageMappingGen.cpp.o):.../swift-build/llvm/tools/clang/lib/CodeGen/CoverageMappingGen.cpp:function clang::CodeGen::CoverageMappingGen::emitEmptyMapping(clang::Decl const*, llvm::raw_ostream&): error: undefined reference to 'llvm::coverage::CoverageMappingWriter::write(llvm::raw_ostream&)'

Once I get the code building error free, I'd like to try and get SourceKit building by default on Linux (see pull request #5903 for an approach that works but is almost certainly the wrong one). When we have the build of SourceKit happening by default on Linux then we can look at reducing the dependencies.

[akyrtzi]

Ok, SGTM.