[SourceKit] Add an in-proc implementation of sourcekitd API functions.

Darwin can use the XPC implementation of this even for the in-proc library
build, this alternative implementation is need for Linux where XPC services
are not available.

This implementation is derived from the existing XPC one but replaces XPC
services' object model with a basic class hierarchy providing
reference-counted objects for the value types and collections that
compose the sourcekitd API's request and response types.

Author: briancroom

tools/SourceKit/tools/sourcekitd/lib/API/CMakeLists.txt

@@ -9,9 +9,14 @@ set(sourcekitdAPI_sources
 
 set(sourcekitdAPI_Darwin_sources
   sourcekitdAPI-XPC.cpp)
-set(LLVM_OPTIONAL_SOURCES ${sourcekitdAPI_Darwin_sources})
+set(sourcekitdAPI_NonDarwin_InProc_sources
+  sourcekitdAPI-InProc.cpp)
+set(LLVM_OPTIONAL_SOURCES ${sourcekitdAPI_Darwin_sources} ${sourcekitdAPI_NonDarwin_InProc_sources})
+
 if(APPLE)
   list(APPEND sourcekitdAPI_sources ${sourcekitdAPI_Darwin_sources})
+elseif(SWIFT_SOURCEKIT_USE_INPROC_LIBRARY)
+  list(APPEND sourcekitdAPI_sources ${sourcekitdAPI_NonDarwin_InProc_sources})
 endif()
 
 add_sourcekit_library(sourcekitdAPI