SourceKit: repair the build when cross-compiling

Not all targets (CMAKE_SYSTEM_NAME) use `lib` and `.a` for the static library
name.  Adjust the name properly for the host that we are building for.  This
repairs the build when performing a cross-compile from Linux to Windows.

Author: compnerd

tools/SourceKit/cmake/modules/AddSwiftSourceKit.cmake

@@ -158,7 +158,7 @@ macro(add_sourcekit_library name)
   set(prefixed_link_libraries)
   foreach(dep ${SOURCEKITLIB_LINK_LIBS})
     if("${dep}" MATCHES "^clang")
-      set(dep "${LLVM_LIBRARY_OUTPUT_INTDIR}/lib${dep}.a")
+      set(dep "${LLVM_LIBRARY_OUTPUT_INTDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}${dep}${CMAKE_STATIC_LIBRARY_SUFFIX}")
     endif()
     list(APPEND prefixed_link_libraries "${dep}")
   endforeach()