Change Expr::isLvalue() to allow the "void" type. This fixes bz2000 submitted by Neil Booth.

Neil, can you point me to the place in the C99 spec that says this is allowed? I thought Expr::isLvalue() conformed to the spec, which says "C99 6.3.2.1: an lvalue is an expression with an object type or an incomplete type other than void.". Please advise.

llvm-svn: 46917