Disable the _StringProcessing import in the Observation module

Fixes rdar://106374161

Author: DougGregor

stdlib/public/Observation/Sources/Observation/CMakeLists.txt

@@ -28,6 +28,7 @@ add_swift_target_library(swiftObservation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS
   SWIFT_COMPILE_FLAGS
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     "-enable-experimental-feature" "Macros"
+    -Xfrontend -disable-implicit-string-processing-module-import
 
   C_COMPILE_FLAGS
     ${swift_runtime_library_compile_flags}