Reinstate "Compile StdlibUnittest with -sil-serialize-all.""

This reinstates commit 79517a8.

Let's try again. All blocking problems should be resolved now.
Compiling StdlibUnittest with -sil-serialize-all gives a much higher test coverage because we can inline and optimize unittest code togeter with the test file.
It already helped to uncover some compiler bugs which we wouldn't have found without this change.

On the downside, a test-run in optimized mode takes considerable longer than before, because some tests need much longer to compile.

Author: eeckstein

stdlib/private/StdlibUnittest/CMakeLists.txt

@@ -48,6 +48,7 @@ add_swift_library(swiftStdlibUnittest SHARED IS_STDLIB
 
   PRIVATE_LINK_LIBRARIES ${swift_stdlib_unittest_private_link_libraries}
   SWIFT_MODULE_DEPENDS ${swift_stdlib_unittest_module_depends}
+  SWIFT_COMPILE_FLAGS -Xfrontend -sil-serialize-all
   FRAMEWORK_DEPENDS ${swift_stdlib_unittest_framework_depends}
   INSTALL_IN_COMPONENT stdlib-experimental)