Perform stricter/more accurate checking for C99 constant expressions

in Sema, per discussion on mailing list.  This doesn't cause any changes 
in the test results.  I'll probably add some more tests at some point, 
but it's an open question what we need to accept to be compatible with 
real code.

This doesn't touch the existing isConstantExpr method on Expr; that 
should be addressed somehow eventually (either removed or refined to 
whatever is appropriate).

llvm-svn: 51318