Export of internal Abseil changes.

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a371d3d4e213c65354dd0e8b1aa28673455aec2a by Matt Armstrong <marmstrong@google.com>:

Fix the C++17 build of absl/types/internal/variant.h.

If ABSL_HAVE_STD_VARIANT is defined this header is now a
nop. While no code should include this header in that case,
the build system may still try to compile it, for example,
if the Bazel parse_headers feature is on.

PiperOrigin-RevId: 210544253
GitOrigin-RevId: a371d3d4e213c65354dd0e8b1aa28673455aec2a
Change-Id: I9c02018b86293b7235fcc9e03fb54f1d86c4b096

absl/types/internal/variant.h

@@ -37,6 +37,8 @@
 #include "absl/types/bad_variant_access.h"
 #include "absl/utility/utility.h"
 
+#if !defined(ABSL_HAVE_STD_VARIANT)
+
 namespace absl {
 
 template <class... Types>
@@ -1612,4 +1614,5 @@ struct VariantHashBase<Variant,
 }  // namespace variant_internal
 }  // namespace absl
 
+#endif  // !defined(ABSL_HAVE_STD_VARIANT)
 #endif  // ABSL_TYPES_variant_internal_H_