Wasm: Set SWIFT_ENABLE_EXPERIMENTAL_STRING_PROCESSING=YES

Because it's been defaulted now

Author: kateinoigakukun

utils/webassembly/build-toolchain.sh

@@ -138,6 +138,8 @@ build_target_toolchain() {
     -D SWIFT_WASI_wasm32_ICU_DATA="$BUILD_SDK_PATH/icu/lib/libicudata.a" \
     -D SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING=YES \
     -D SWIFT_ENABLE_EXPERIMENTAL_DISTRIBUTED=YES \
+    -D SWIFT_ENABLE_EXPERIMENTAL_STRING_PROCESSING=YES \
+    -D EXPERIMENTAL_STRING_PROCESSING_SOURCE_DIR="$SOURCE_PATH/swift-experimental-string-processing" \
     -G Ninja \
     -S "$SOURCE_PATH/swift"