[CMake] Add SWIFT_STDLIB_SHORT_MANGLING_LOOKUPS to StandaloneOverlay

This will allow it to have a sensible default when e.g. building the
back deploy concurrency library.

Addresses rdar://85815655

Author: edymtt

cmake/modules/StandaloneOverlay.cmake

@@ -82,6 +82,9 @@ set(SWIFT_DARWIN_MODULE_ARCHS "" CACHE STRING
 targets on Darwin platforms. These targets are in addition to the full \
 library targets.")
 
+option(SWIFT_STDLIB_SHORT_MANGLING_LOOKUPS
+       "Build stdlib with fast-path context descriptor lookups based on well-known short manglings."
+       TRUE)
 
 # -----------------------------------------------------------------------------
 # Constants