[vs2017] Adapt some variadic template aliases to not trip MSVC 2017. (#37284)
[vs2017] Adapt some variadic template aliases to not trip MSVC 2017. (#37284)

It seems that MSVC 2017 trips parsing an using of a using of a variadic
template. Removing one level of using seems to work fine. A preprocessor
macro allows to keep using the same syntax in both MSVC 2017 and other
compilers without making a lot of a mess.

I think this might have been uncovered by landing apple/llvm-project#2898
when it was picked up by the auto-merger for the swift/main branch.
I think this was not a problem until now, because Metadata.h was
compiled using the just compiled Clang until now. LLDB is compiled using
MSVC in Windows.
atrick/swift,xwu/swift,atrick/swift,parkera/swift,glessard/swift,hooman/swift,JGiola/swift,apple/swift,ahoppen/swift,tkremenek/swift,tkremenek/swift,parkera/swift,rudkx/swift,rudkx/swift,parkera/swift,apple/swift,apple/swift,xwu/swift,parkera/swift,roambotics/swift,gregomni/swift,hooman/swift,benlangmuir/swift,ahoppen/swift,xwu/swift,JGiola/swift,roambotics/swift,glessard/swift,parkera/swift,apple/swift,JGiola/swift,rudkx/swift,rudkx/swift,atrick/swift,tkremenek/swift,tkremenek/swift,roambotics/swift,glessard/swift,xwu/swift,ahoppen/swift,rudkx/swift,gregomni/swift,benlangmuir/swift,hooman/swift,JGiola/swift,ahoppen/swift,JGiola/swift,apple/swift,xwu/swift,rudkx/swift,hooman/swift,gregomni/swift,hooman/swift,roambotics/swift,hooman/swift,benlangmuir/swift,parkera/swift,roambotics/swift,gregomni/swift,atrick/swift,gregomni/swift,xwu/swift,parkera/swift,glessard/swift,parkera/swift,roambotics/swift,xwu/swift,benlangmuir/swift,tkremenek/swift,JGiola/swift,atrick/swift,tkremenek/swift,tkremenek/swift,hooman/swift,glessard/swift,glessard/swift,apple/swift,atrick/swift,ahoppen/swift,ahoppen/swift,gregomni/swift,benlangmuir/swift,benlangmuir/swift