Allow scoped extern(C) declarations

[rsaarelm]

This lets you write FFI functions like

void foo() {  // Neat function in module namespace
  extern(C) void foo();  // C function with same name in wrapper implementation namespace
  ...
}

[FeepingCreature]

Yeah extern(C)s should really be declarations probably. That whole piece of compiler machinery was added after extern declarations, which is why you can declare structs in a function but not externs.