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).

llvm-svn: 41476
diff --git a/clang/test/Sema/compare.c b/clang/test/Sema/compare.c
index 0aeeb30..d4e29e8 100644
--- a/clang/test/Sema/compare.c
+++ b/clang/test/Sema/compare.c
@@ -6,3 +6,7 @@
   return C != 0;
 }
 
+int equal(char *a, const char *b)
+{
+    return a == b;
+}