Add missing ABSL_DLL for a few functions

[zcbenz]

The throw_bad_optional_access and ThrowBadVariantAccess are used in a few inlined functions and should have the ABSL_DLL prefix, otherwise linking with absl.dll will fail with errors like:

error LNK2019: unresolved external symbol "void __cdecl absl::
optional_internal::throw_bad_optional_access(void)" (?throw_bad_optional_access@opt
ional_internal@absl@@YAXXZ) referenced in function "public: class base::TimeDelta &
 __cdecl absl::optional<class base::TimeDelta>::value(void)& " (?value@?$optional@V
TimeDelta@base@@@absl@@QEGAAAEAVTimeDelta@base@@XZ)\r\n'

[derekmauro]

Thank you. This should be merged soon.