Use an inline namespace for Abseil.

This prevents Drake's build of Abseil from interfering with a user's
downstream build of Abseil when the user is using static linking.

--- absl/base/options.h
+++ absl/base/options.h
@@ -205,8 +205,8 @@
 // be changed to a new, unique identifier name.  In particular "head" is not
 // allowed.
 
-#define ABSL_OPTION_USE_INLINE_NAMESPACE 0
-#define ABSL_OPTION_INLINE_NAMESPACE_NAME head
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME drake_vendor
 
 // ABSL_OPTION_HARDENED
 //

--- absl/debugging/symbolize_elf.inc
+++ absl/debugging/symbolize_elf.inc
@@ -1653,9 +1653,11 @@
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#if 0  // This (dead) code disobeys the drake_vendor inline namespace.
 extern "C" bool AbslInternalGetFileMappingHint(const void **start,
                                                const void **end, uint64_t *offset,
                                                const char **filename) {
   return absl::debugging_internal::GetFileMappingHint(start, end, offset,
                                                       filename);
 }
+#endif
