NCGenerics: handle failure to load stdlib
The stdlib module can end up being a non-null ModuleDecl that contains
no files, due to a failure to load the stdlib.
In such an unusual case, fall-back to synthesizing Copyable/Escapable
into the Builtin module so that Sema can proceed.
rdar://129092011
(cherry picked from commit b02b94cd526a7c57db5b5f532535f35c51e36f7f)