Revisit `BasicSpecifiers::External` for user-authored `extern`

[cdacamar]

Revisit this one the IFC has a better representation for the syntactic extern on function declarations vs compiler-generated/implicit external-linkage.

e.g.

extern void f();
void g();

On MSVC, both of the functions above have external-linkage, but BasicSpecifiers::External is insufficient for telling us whether the user wrote extern or external-linkage was generated by the compiler.