Moving compiler-src-dir refs

Moving the rest of the references to the compiler source directory to
use the `SwiftCore_SWIFTC_SOURCE_DIR` variable instead of
`${PROJECT_SOURCE_DIR}/../../`.

Author: etcwilde

Runtimes/Core/Threading/CMakeLists.txt

@@ -3,8 +3,8 @@
 #       and linking empty objects.
 #       https://github.com/swiftlang/swift/issues/77435
 add_library(swiftThreading OBJECT
-  "${PROJECT_SOURCE_DIR}/../../lib/Threading/C11.cpp"
-  "${PROJECT_SOURCE_DIR}/../../lib/Threading/Linux.cpp"
-  "${PROJECT_SOURCE_DIR}/../../lib/Threading/Pthreads.cpp"
-  "${PROJECT_SOURCE_DIR}/../../lib/Threading/Win32.cpp")
+  "${SwiftCore_SWIFTC_SOURCE_DIR}/lib/Threading/C11.cpp"
+  "${SwiftCore_SWIFTC_SOURCE_DIR}/lib/Threading/Linux.cpp"
+  "${SwiftCore_SWIFTC_SOURCE_DIR}/lib/Threading/Pthreads.cpp"
+  "${SwiftCore_SWIFTC_SOURCE_DIR}/lib/Threading/Win32.cpp")
 target_link_libraries(swiftThreading PRIVATE swiftShims)

Runtimes/Core/cmake/modules/AvailabilityMacros.cmake

@@ -1,4 +1,4 @@
-file(STRINGS "${PROJECT_SOURCE_DIR}/../../utils/availability-macros.def" availability_defs)
+file(STRINGS "${SwiftCore_SWIFTC_SOURCE_DIR}/utils/availability-macros.def" availability_defs)
 list(FILTER availability_defs EXCLUDE REGEX "^\\s*(#.*)?$")
 foreach(def ${availability_defs})
   add_compile_options("$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -define-availability -Xfrontend \"${def}\">")

Runtimes/Core/cmake/modules/gyb.cmake

@@ -23,7 +23,7 @@ function(gyb_expand source output)
   endforeach()
   list(REMOVE_DUPLICATES gyb_expand_deps)
 
-  set(utils_dir "${PROJECT_SOURCE_DIR}/../../utils/")
+  set(utils_dir "${SwiftCore_SWIFTC_SOURCE_DIR}/utils/")
   set(gyb_tool "${utils_dir}/gyb")
 
   # All the tidbits to track for changes

Runtimes/Core/stubs/CMakeLists.txt

@@ -45,7 +45,7 @@ target_include_directories(swiftStdlibStubs PRIVATE
   #   swift/Runtime/Debug.h
   # ...
   #
-  "${PROJECT_SOURCE_DIR}/../../include")
+  "${SwiftCore_SWIFTC_SOURCE_DIR}/include")
 
 if(APPLE)
   set_property(SOURCE SwiftNativeNSXXXBaseARC.m