unittests: attempt to repair the unified build

This is a speculative fix for the unified build which was broken by
PR #64177 as the `CMAKE_SOURCE_DIR` is going to point to LLVM not Swift.

Author: compnerd

unittests/ClangImporter/CMakeLists.txt

@@ -1,4 +1,5 @@
-configure_file(${CMAKE_SOURCE_DIR}/stdlib/public/Cxx/std/libstdcxx.modulemap ${CMAKE_CURRENT_BINARY_DIR}/libstdcxx.modulemap COPYONLY)
+configure_file(${SWIFT_SOURCE_DIR}/stdlib/public/Cxx/std/libstdcxx.modulemap
+  ${CMAKE_CURRENT_BINARY_DIR}/libstdcxx.modulemap COPYONLY)
 
 add_swift_unittest(SwiftClangImporterTests
   ClangImporterTests.cpp