AST: Remove dead-code/update reference to standard

GetGVALinkageForFunction handles TSK_ExplicitInstantiationDeclaration
twice, remove the redundant code trying to handle it again.

While we are here, update the reference we make to the standard.  It
seems like another paragraph was added causing this text to get
renumbered.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208850 91177308-0d34-0410-b5e6-96231b3b80d8