[clang][Interp] Allow evaluating standalone complex expressions

[tbaeder]

Summary

We reach visitExpr() when evaluating standalone expressions. However,
the RetValue() code path was unused before, because we never reach it,
even with structs or arrays.

RetValue expects a pointer on the stack it can take apart to return an
APValue, so provide it with one.

[tbaeder]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

2287

I added these changes to make the test work, but they are thematically different from the purpose of this patch. However, without them, the test doesn't work.

[aaron.ballman]

LGTM, but you should change the summary to Allow evaluating standalone complex expressions when landing.