AST: Ensure implicit records have default visibility

Types composed with certain implicit record types would have their RTTI
marked as hidden because the implicit record type didn't have any
visibility.

This manifests itself as triggering false positives from tools like
clang's -fsantize=function feature.  The RTTI for a function type's
return type wouldn't match if the return type was an implicit record
type.

Patch by Stephan Bergmann!

llvm-svn: 226148