Add a build-script flag for enabling copy-on-write existentials

Author: aschwaighofer

utils/build-script-impl

@@ -253,6 +253,7 @@ KNOWN_SETTINGS=(
     coverage-db                        ""        "If set, coverage database to use when prioritizing testing"
     build-toolchain-only               ""        "If set, only build the necessary tools to build an external toolchain"
     skip-local-host-install            ""        "If we are cross-compiling multiple targets, skip an install pass locally if the hosts match"
+    swift-runtime-enable-cow-existentials "0"    "Enable the copy-on-write existential implementation"
 )
 
 # Centralized access point for traced command invocation.
@@ -2116,6 +2117,7 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DSWIFT_STDLIB_BUILD_TYPE:STRING="${SWIFT_STDLIB_BUILD_TYPE}"
                     -DSWIFT_STDLIB_ASSERTIONS:BOOL=$(true_false "${SWIFT_STDLIB_ENABLE_ASSERTIONS}")
                     -DSWIFT_STDLIB_ENABLE_RESILIENCE:BOOL=$(true_false "${SWIFT_STDLIB_ENABLE_RESILIENCE}")
+                    -DSWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS:BOOL=$(true_false "${SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS}")
                     -DSWIFT_STDLIB_USE_NONATOMIC_RC:BOOL=$(true_false "${SWIFT_STDLIB_USE_NONATOMIC_RC}")
                     -DSWIFT_STDLIB_SIL_SERIALIZE_ALL:BOOL=$(true_false "${SWIFT_STDLIB_SIL_SERIALIZE_ALL}")
                     -DSWIFT_NATIVE_LLVM_TOOLS_PATH:STRING="${native_llvm_tools_path}"