Fix a gcc warning.

In r181677 I removed this llvm_unreachable and it introduced a gcc
warning. Add it back.

Thanks to Patrik Hägglund for noticing it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181704 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 9901080..49e3639 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -7838,6 +7838,8 @@
   case TSK_ImplicitInstantiation:
     return GVA_TemplateInstantiation;
   }
+
+  llvm_unreachable("Invalid Linkage!");
 }
 
 bool ASTContext::DeclMustBeEmitted(const Decl *D) {