build: canonicalise source path before use (NFC)

If the source tree is checked out into a patch behind a symlink, the path can
get expanded and fail tests.  Be more resilient by canonicalise the path prior
to use.

Author: compnerd

CMakeLists.txt

@@ -497,7 +497,7 @@ else()
   swift_common_unified_build_config(SWIFT)
 endif()
 
-set(SWIFT_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
+get_filename_component(SWIFT_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR} REALPATH)
 set(SWIFT_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}")
 set(SWIFT_CMAKE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 set(SWIFT_MAIN_INCLUDE_DIR "${SWIFT_SOURCE_DIR}/include")