Speculative build fix for lld on Windows

I can't access my Windows machine at the moment, but the build was breaking with:

c:\b\build\slave\crwinclanglld\build\src\third_party\llvm\tools\lld\lib\readerwriter\elf\mips\MipsTargetLayout.h(38) : error C2440: 'return' : cannot convert from 'lld::elf::MipsGOTSection<ELFT> *' to 'lld::elf::AtomSection<ELFT> *'
        with
        [
            ELFT=lld::elf::Mips32ELType
        ]
        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

I think this regressed caused in 234727: that the forward-declarations
of MipsGOTSection<> is not enough.

llvm-svn: 234736