Removed bogus "return true" in Expr::isConstantExpr that returned true for all

expressions. This appears to be a regression introduced in r51113 that caused
many test cases to fail (there is still a test case in the Analysis directory
that is failing):

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20080512/005706.html

llvm-svn: 51164