Subsume SWIFT_STDLIB_USE_NONATOMIC_RC into SWIFT_STDLIB_SINGLE_THREADED_RUNTIME (#33643)

utils/build-script-impl

@@ -186,7 +186,6 @@ KNOWN_SETTINGS=(
     swift-primary-variant-sdk                     ""                "default SDK for target binaries"
     swift-runtime-enable-leak-checker             "0"               "Enable leaks checking routines in the runtime"
     swift-stdlib-enable-assertions                "1"               "enable assertions in Swift"
-    swift-stdlib-use-nonatomic-rc                 "0"               "build the Swift stdlib and overlays with nonatomic reference count operations enabled"
     swift-tools-enable-lto                        ""                "enable LTO compilation of Swift tools. *NOTE* This does not include the swift standard library and runtime. Must be set to one of 'thin' or 'full'"
     extra-swift-args                              ""                "Extra arguments to pass to swift modules which match regex. Assumed to be a flattened cmake list consisting of [module_regexp, args, module_regexp, args, ...]"
     report-statistics                             "0"               "set to 1 to generate compilation statistics files for swift libraries"
@@ -1755,7 +1754,6 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DSWIFT_ANALYZE_CODE_COVERAGE:STRING=$(toupper "${SWIFT_ANALYZE_CODE_COVERAGE}")
                     -DSWIFT_STDLIB_BUILD_TYPE:STRING="${SWIFT_STDLIB_BUILD_TYPE}"
                     -DSWIFT_STDLIB_ASSERTIONS:BOOL=$(true_false "${SWIFT_STDLIB_ENABLE_ASSERTIONS}")
-                    -DSWIFT_STDLIB_USE_NONATOMIC_RC:BOOL=$(true_false "${SWIFT_STDLIB_USE_NONATOMIC_RC}")
                     -DSWIFT_ENABLE_COMPATIBILITY_OVERRIDES:BOOL=$(true_false "${SWIFT_ENABLE_COMPATIBILITY_OVERRIDES}")
                     -DSWIFT_STDLIB_SINGLE_THREADED_RUNTIME:BOOL=$(true_false "${SWIFT_STDLIB_SINGLE_THREADED_RUNTIME}")
                     -DSWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS:BOOL=$(true_false "${SWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS}")