swift234

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

This reinstates commit 79517a8edfb6a8734c6b3025d76cc54c8d157a6a.

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.