[wasm] Emit libTesting.a objects with single-threaded LLVM codegen unit

Since 5f2b002, swift-testing is being
compiled with WMO, which removes some of inter-object references in
object files by DCE. The inter-object reference removal revealed a
long-standing issue that the runtime metadata sections of objects in an
archive are not always included in the final binary if symbols from
those objects are not referenced anywhere. To force including all
metadata sections in the final binary, we have to emit everything in a
single object file when building the archive.
This issue happens only for Wasm SDK, which ships swift-testing as a
static archive.

Author: kateinoigakukun

utils/swift_build_support/swift_build_support/products/wasmswiftsdk.py

@@ -69,9 +69,12 @@ def _build_swift_testing(self, swift_host_triple, short_triple, wasi_sysroot):
             'CMAKE_Swift_COMPILER_TARGET', swift_host_triple)
         swift_testing.cmake_options.define('CMAKE_SYSROOT', wasi_sysroot)
         swift_resource_dir = os.path.join(dest_dir, 'usr', 'lib', 'swift_static')
+        # For statically linked objects in an archive, we have to use singlethreaded
+        # LLVM codegen unit to prevent runtime metadata sections from being stripped
+        # at link-time.
         swift_testing.cmake_options.define(
             'CMAKE_Swift_FLAGS',
-            f'-sdk {wasi_sysroot} -resource-dir {swift_resource_dir}')
+            f'-sdk {wasi_sysroot} -resource-dir {swift_resource_dir} -Xfrontend -enable-single-module-llvm-emission')
         clang_resource_dir = os.path.join(dest_dir, 'usr', 'lib', 'clang')
         swift_testing.cmake_options.define(
             'CMAKE_CXX_FLAGS', f'-resource-dir {clang_resource_dir}')