Revert r363191 "[MS] Pretend constexpr variable template specializations are inline"

The next Visual Studio update will fix this issue, and it doesn't make
sense to implement this non-conforming behavior going forward.

llvm-svn: 364476
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index d7778a71..db8e74e 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -9809,25 +9809,10 @@
     return StrongLinkage;
 
   case TSK_ExplicitSpecialization:
-    if (Context.getTargetInfo().getCXXABI().isMicrosoft()) {
-      // If this is a fully specialized constexpr variable template, pretend it
-      // was marked inline. MSVC 14.21.27702 headers define _Is_integral in a
-      // header this way, and we don't want to emit non-discardable definitions
-      // of these variables in every TU that includes <type_traits>. This
-      // behavior is non-conforming, since another TU could use an extern
-      // template declaration for this variable, but for constexpr variables,
-      // it's unlikely for a user to want to do that. This behavior can be
-      // removed if the headers change to explicitly mark such variable template
-      // specializations inline.
-      if (isa<VarTemplateSpecializationDecl>(VD) && VD->isConstexpr())
-        return GVA_DiscardableODR;
-
-      // Use ODR linkage for static data members of fully specialized templates
-      // to prevent duplicate definition errors with MSVC.
-      if (VD->isStaticDataMember())
-        return GVA_StrongODR;
-    }
-    return StrongLinkage;
+    return Context.getTargetInfo().getCXXABI().isMicrosoft() &&
+                   VD->isStaticDataMember()
+               ? GVA_StrongODR
+               : StrongLinkage;
 
   case TSK_ExplicitInstantiationDefinition:
     return GVA_StrongODR;