[C++20] [Modules] Handling template declare with debug info

It looks an overlook that debug info can't play well with
explicit template instantiation. Tested in donwstream for years. I just
forgot to upstream it.

Author: ChuanqiXu9

clang/lib/CodeGen/CGVTables.cpp

@@ -1138,7 +1138,9 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
                  llvm::Function::InternalLinkage;
 
       case TSK_ExplicitInstantiationDeclaration:
-        llvm_unreachable("Should not have been asked to emit this");
+        return IsExternalDefinition
+                   ? llvm::GlobalVariable::AvailableExternallyLinkage
+                   : llvm::GlobalVariable::ExternalLinkage;
       }
   }
 

clang/test/Modules/template-declare.cppm

@@ -0,0 +1,39 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -dwarf-version=4 -debug-info-kind=constructor \
+// RUN:     -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -dwarf-version=4 -debug-info-kind=constructor \
+// RUN:     -emit-module-interface -o %t/b.pcm -fmodule-file=a=%t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cpp -dwarf-version=4 -debug-info-kind=constructor \
+// RUN:     -emit-llvm -o - -fmodule-file=a=%t/a.pcm -fmodule-file=b=%t/b.pcm | FileCheck %t/b.cpp 
+
+//--- a.cppm
+export module a;
+export template <class T>
+class a {
+private:
+    T *data;
+
+public:
+    virtual T* getData();
+};
+
+extern template class a<char>;
+
+//--- b.cppm
+export module b;
+import a;
+export struct b {
+    a<char> v;
+};
+
+//--- b.cpp
+module b;
+extern "C" void func() {
+    b();
+}
+
+// It is fine enough to check that we won't crash.
+// CHECK: define {{.*}}void @func()