Tie Swift backtracing pieces together in build

This patch ties the runtime and stdlib backtracing pieces together.
The `__swift_isThunkFunction` SPI is provided in the backtracing
runtime, which is gated behind `SWIFT_ENABLE_BACKTRACING`. If that flag
is turned off, the `Backtracing` part of the stdlib fails to build
because the symbols are missing, causing a build failure.

I've also added the explicit `option(SWIFT_ENABLE_BACKTRACING` option so
that incremental re-runs of CMake get the flag added with a nice
comment and a boolean value. The option is defaulted to OFF, though
build-script is defaulting it to ON.

Author: etcwilde

CMakeLists.txt

@@ -647,6 +647,10 @@ option(SWIFT_ENABLE_MACCATALYST
   "Build the Standard Library and overlays with MacCatalyst support"
   FALSE)
 
+option(SWIFT_ENABLE_BACKTRACING
+  "Build backtracing runtime support"
+  FALSE)
+
 set(SWIFT_DARWIN_DEPLOYMENT_VERSION_MACCATALYST "14.5" CACHE STRING
   "Minimum deployment target version for macCatalyst")
 

stdlib/public/CMakeLists.txt

@@ -159,7 +159,9 @@ if(SWIFT_BUILD_STDLIB)
     add_subdirectory(Observation)
   endif()
 
-  add_subdirectory(Backtracing)
+  if(SWIFT_ENABLE_BACKTRACING)
+    add_subdirectory(Backtracing)
+  endif()
 endif()
 
 if(SWIFT_BUILD_REMOTE_MIRROR)