[ThinLTO] WeakAny fixes/cleanup

Ensure WeakAny variables are imported as ExternalWeak declarations. To
handle WeakAny more consistently and fix this issue:

1) Update helper doImportAsDefinition to properly flag WeakAny variables
   and aliases as not importing defintions.

   Update callers of doImportAsDefinition to remove now redundant checks for
   WeakAny aliases, or ignore aliases, as appropriate.

2) Add any !doImportAsDefinition GVs to DoNotLinkFromSource set during
   linking of the GV prototype, where we usually add GVs to the
   DoNotLinkFromSource set for other reasons.

   Remove now unnecessary adding of WeakAny aliases to
   DoNotLinkFromSource set from copyGlobalAliasProto.

   Remove now unnecessary guard against linking non-imported function
   bodies from ModuleLinker::run.

llvm-svn: 252626