Export of internal Abseil changes.

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f6241e6923d4d1525d3ec54bc5b85178b18612e9 by Derek Mauro <dmauro@google.com>:

Make implicit_cast constexpr.
GitHub #191

PiperOrigin-RevId: 217726953
GitOrigin-RevId: f6241e6923d4d1525d3ec54bc5b85178b18612e9
Change-Id: I46e4d92cbd54a38bfe5faeb939cf19d871d8b697

absl/base/casts.h

@@ -105,7 +105,7 @@ struct is_bitcastable
 //
 //
 // Such implicit cast chaining may be useful within template logic.
 // Such implicit cast chaining may be useful within template logic.
 template <typename To>
 template <typename To>
-inline To implicit_cast(typename absl::internal::identity_t<To> to) {
+constexpr To implicit_cast(typename absl::internal::identity_t<To> to) {
   return to;
   return to;
 }
 }