Eliminate the oddly-named Sema::ComparePropertiesInBaseAndSuper, which
did a redundant traversal of the lexical declarations in the
superclass. Instead, when we declare a new property, look into the
superclass to see whether we're redeclaring the property. Goot for 1%
of -fsyntax-only time on Cocoa.h and a little less than 3% on my
modules test case.