Revert "[analyzer] Treat all struct values as regions (even rvalues)."

This turned out to have many implications, but what eventually seemed to
make it unworkable was the fact that we can get struct values (as
LazyCompoundVals) from other places besides return-by-value function calls;
that is, we weren't actually able to "treat all struct values as regions"
consistently across the entire analyzer core.

Hopefully we'll be able to come up with an alternate solution soon.

This reverts r163066 / 02df4f0aef142f00d4637cd851e54da2a123ca8e.

llvm-svn: 163218