Make Badge constructors noexcept

Summary:
Make `folly::badge` and `folly::any_badge` noexcept constructible.
Cannot make constructors explicitly defaulted due to classes becoming aggregate types.
Aggregate type construction takes precedence over value constructors so we can construct them anywhere:
```
class A{
    A() = default;
};

int main() {
    A a = {};
}
```
Cannot mark them `explicit` either, since we prefer to have conversions auto-magical where possible.

Reviewed By: yfeldblum

Differential Revision: D31983342

fbshipit-source-id: ddeffc30e6332882a490afea5bf023522f96c18e

folly/lang/Badge.h

@@ -51,7 +51,7 @@ namespace folly {
 template <typename Holder>
 class badge {
   friend Holder;
-  badge() {}
+  badge() noexcept {}
 };
 
 /**
@@ -77,7 +77,7 @@ class any_badge {
   template <
       typename Holder,
       std::enable_if_t<folly::IsOneOf<Holder, Holders...>::value, int> = 0>
-  /* implicit */ any_badge(badge<Holder>) {}
+  /* implicit */ any_badge(badge<Holder>) noexcept {}
 };
 
 } // namespace folly