Remove the type traits UTT_IsLvalueExpr and UTT_IsRvalueExpr.

As might be surmised from their names, these aren't type traits, they're
expression traits. Amazingly enough, they're expression traits that we
have, and fully implement. These "type" traits are even parsed from the
same tokens as the expression traits. Luckily, the parser only tried the
expression trait parsing for these tokens, so this was all just a pile
of dead code.

llvm-svn: 130643