[analyzer] Reuse a LazyCompoundVal if its type matches the new region.

This allows us to keep from chaining LazyCompoundVals in cases like this:
  CGRect r = CGRectMake(0, 0, 640, 480);
  CGRect r2 = r;
  CGRect r3 = r2;

Previously we only made this optimization if the struct did not begin with
an aggregate member, to make sure that we weren't picking up an LCV for
the first field of the struct. But since LazyCompoundVals are typed, we can
make that inference directly by comparing types.

This is a pure optimization; the test changes are to guard against possible
future regressions.
Re-apply "[analyzer] Model trivial copy/move ctors with an aggregate bind."

With the optimization in the previous commit, this should be safe again.

Originally applied in r173951, then reverted in r174069.
[analyzer] Explain why we have system-header-simulator*.h files.

Suggested by Csaba. Text based on an e-mail of mine on cfe-dev.