compiler: don't generate stubs for ambiguous direct interface methods

compiler: don't generate stubs for ambiguous direct interface methods

Current implementation checks whether it has to generate a stub method for a
promoted method of an embedded struct field in Type::build_stub_methods(). If
the promoted method is ambiguous it's simply skipped. But struct types that
can fit in an interface value (e.g. structs that consist of a single pointer
field) get a second chance in Type::build_direct_iface_stub_methods().

This patch adds the same check used by Type::build_stub_methods() to
Type::build_direct_iface_stub_methods().

Fixes golang/go#52870

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/405974