[sourcekitd] Use add_llvm_symbol_exports instead of our old copy

[benlangmuir]

Reduces duplication with llvm and, more importantly, gives us Linux
support via a linker script. This will let us link libsourcekitdInProc
in binaries that have their own copy of llvm symbols on Linux, and let
sourcekit-lsp stop using dlopen with RTLD_DEEPBIND, which fixes
running with asan among other things.

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please build toolchain

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e96f98b

[swift-ci]

Linux Toolchain (Ubuntu 16.04)
Download Toolchain
Git Sha - e96f98b

Install command
tar zxf swift-PR-31287-363-ubuntu16.04.tar.gz
More info

[benlangmuir]

@swift-ci please build toolchain macOS

[benlangmuir]

@swift-ci please test macOS

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e96f98b

[compnerd]

The macOS failure is due to an incorrect automerge in lldb fixed by @fredriss on Friday.

/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/llvm-project/lldb/source/Plugins/ExpressionParser/Swift/SwiftREPL.cpp:570:32: error: no type named 'ImplicitModuleImportKind' in 'swift::SourceFile'
      const swift::SourceFile::ImplicitModuleImportKind implicit_import_kind =
            ~~~~~~~~~~~~~~~~~~~^
/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/llvm-project/lldb/source/Plugins/ExpressionParser/Swift/SwiftREPL.cpp:571:30: error: no member named 'ImplicitModuleImportKind' in 'swift::SourceFile'
          swift::SourceFile::ImplicitModuleImportKind::Stdlib;
          ~~~~~~~~~~~~~~~~~~~^
2 errors generated.

Lets give this another shot.

[compnerd]

@swift-ci please test macOS platform

[compnerd]

@swift-ci please build toolchain macOS

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - e96f98b

Install command
tar -zxf swift-PR-31287-504-osx.tar.gz --directory ~/