Enable "Macros" feature in stdlib/Observation for older compilers

Author: DougGregor

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

@@ -27,6 +27,7 @@ add_swift_target_library(swiftObservation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS
 
   SWIFT_COMPILE_FLAGS
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+    "-enable-experimental-feature" "Macros"
 
   C_COMPILE_FLAGS
     ${swift_runtime_library_compile_flags}

stdlib/public/core/CMakeLists.txt

@@ -289,6 +289,8 @@ endif()
 # STAGING: Temporarily avoids having to write #fileID in Swift.swiftinterface.
 list(APPEND swift_stdlib_compile_flags "-Xfrontend" "-enable-experimental-concise-pound-file")
 
+list(APPEND swift_stdlib_compile_flags "-enable-experimental-feature" "Macros")
+
 if(SWIFT_CHECK_ESSENTIAL_STDLIB)
   add_swift_target_library(swift_stdlib_essential ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB IS_STDLIB_CORE
       INSTALL_IN_COMPONENT never_install