Fix bogus warnings (noticed by Chris) with array-constraints.c.

Remove bogus type conversions in Sema::GetTypeForDeclarator(). This commit
only deals with the array types (DeclaratorCheck::Array), though the
rest of this routine should be reviewed. Given the complexity of C declarators,
I don't want to change the entire routine now (will discuss with Chris tomorrow).

llvm-svn: 41443