Get rid of -Wno-uninitialized

Summary: This is a pretty useful warning. Turn it back on and fix the fallout.
It's amazingly limited damage, considering what usually happens when
warnings aren't turned on.

Reviewed By: @markw65

Differential Revision: D1547861
Make Type::rawVal() work whenever Type::isConst() returns true

Summary: And clean up a number of places that had special handling for cases rawVal() didn't work despite isConst() being true.

Also, kill hasRawVal().

Reviewed By: @edwinsmith, @alexmalyshev, @bertmaher

Differential Revision: D1548617