Add Type::getAsBuiltinType() and Type::builtinTypesAreCompatible().

Add Type::getAsBuiltinType() and Type::builtinTypesAreCompatible().
Modified Type::typesAreCompatible() to use the above.

This fixes the following bug submitted by Keith Bauer (thanks!).

int equal(char *a, const char *b)
{
    return a == b;
}

Also tweaked Sema::CheckCompareOperands() to ignore the qualifiers when
comparing two pointer types (though it doesn't relate directly to this bug).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41476 91177308-0d34-0410-b5e6-96231b3b80d8