[WinEH] Don't emit state stores or EH thunks for available_externally functions

[WinEH] Don't emit state stores or EH thunks for available_externally functions

The exception handler thunk needs to reference the LSDA of the parent
function, which won't be emitted if it's available_externally.

Fixes PR35736. ThinLTO ends up producing available_externally functions
that use _CxxFrameHandler3.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321532 91177308-0d34-0410-b5e6-96231b3b80d8