Set up a client-side C library to talk with C APIs exposed from the compiler

[nkcsgexi]

We could use functions like dlopen and dlsym in this client-side library to talk with
compiler-side C APIs.

[nkcsgexi]

@swift-ci please test

[cltnschlosser]

Is the idea to use this for llvm?

[artemcm]

When I try to build this with CMake, I get:

CMake Error at Sources/CMakeLists.txt:9 (add_subdirectory):                                                                                                                                                                              The source directory                                                                                                                                                                                                                                                                                                                                                                                                                                                            /Volumes/Data/GHWorkspace1/swift-driver/Sources/_CSwiftDriver/src

  does not contain a CMakeLists.txt file.

Is there perhaps a file missing from this PR?
This target's CMakeLists should define it as a library, and we need to take care to link it in statically when building libSwiftDriver to avoid having to install another shared library into the toolchain.

[artemcm]

Is the idea to use this for llvm?

@cltnschlosser, something like this will be an example of where this is useful:
swiftlang/swift#34786

[nkcsgexi]

@cltnschlosser, this is to allow libSwiftDriver to talk with Swift compiler from different toolchains. In that case, we cannot statically link against libraries vendored from a compiler. We need to rely on C APIs like dlopen and dlsym to load dylibs dynamically.

[nkcsgexi]

@artemcm hmm, will take a look at the cmake issue.

[tschuett]

sourcekit-lsp does this already:
https://github.com/apple/sourcekit-lsp/blob/eaea937ee4f5aea0a055bd5f682f31da0fa7f82f/Sources/SourceKitD/SourceKitDImpl.swift#L64
and
https://github.com/apple/sourcekit-lsp/blob/eaea937ee4f5aea0a055bd5f682f31da0fa7f82f/Sources/SourceKitD/sourcekitd_functions.swift#L16

Move some shared code to TSC?

[nkcsgexi]

@tschuett yeah, refactoring those pieces seem to me a good idea. Regardless of whether we need to invoke dlopen in C, we still need to set up this C target to host those compiler headers for the dependencies scanning library.

[tschuett]

No worries. You are missing some newlines at the end of files in a couple of places.

[nkcsgexi]

@swift-ci please test