Explicitly initialize StreamingDiagnostic in derived class copy ctors

To pacify a GCC warning:

[1/1] Building CXX object tools/clang/lib/Analysis/CMakeFiles/obj.clangAnalysis.dir/Dominators.cpp.o
In file included from /work/llvm.monorepo/clang/include/clang/AST/NestedNameSpecifier.h:18:0,
                 from /work/llvm.monorepo/clang/include/clang/AST/Type.h:21,
                 from /work/llvm.monorepo/clang/include/clang/AST/DeclarationName.h:16,
                 from /work/llvm.monorepo/clang/include/clang/AST/DeclBase.h:18,
                 from /work/llvm.monorepo/clang/include/clang/Analysis/AnalysisDeclContext.h:20,
                 from /work/llvm.monorepo/clang/include/clang/Analysis/Analyses/Dominators.h:16,
                 from /work/llvm.monorepo/clang/lib/Analysis/Dominators.cpp:9:
/work/llvm.monorepo/clang/include/clang/Basic/Diagnostic.h:
In copy constructor ‘clang::DiagnosticBuilder::DiagnosticBuilder(const clang::DiagnosticBuilder&)’:
/work/llvm.monorepo/clang/include/clang/Basic/Diagnostic.h:1287:3:
warning: base class ‘class clang::StreamingDiagnostic’ should be explicitly initialized in the copy constructor [-Wextra]
   DiagnosticBuilder(const DiagnosticBuilder &D) {
   ^
In file included from /work/llvm.monorepo/clang/include/clang/AST/Type.h:29:0,
                 from /work/llvm.monorepo/clang/include/clang/AST/DeclarationName.h:16,
                 from /work/llvm.monorepo/clang/include/clang/AST/DeclBase.h:18,
                 from /work/llvm.monorepo/clang/include/clang/Analysis/AnalysisDeclContext.h:20,
                 from /work/llvm.monorepo/clang/include/clang/Analysis/Analyses/Dominators.h:16,
                 from /work/llvm.monorepo/clang/lib/Analysis/Dominators.cpp:9:
/work/llvm.monorepo/clang/include/clang/Basic/PartialDiagnostic.h:
In copy constructor ‘clang::PartialDiagnostic::PartialDiagnostic(const clang::PartialDiagnostic&)’:
/work/llvm.monorepo/clang/include/clang/Basic/PartialDiagnostic.h:52:3:
warning: base class ‘class clang::StreamingDiagnostic’ should be explicitly initialized in the copy constructor [-Wextra]
   PartialDiagnostic(const PartialDiagnostic &Other) : DiagID(Other.DiagID) {
   ^

clang/include/clang/Basic/Diagnostic.h

@@ -1284,7 +1284,7 @@ class DiagnosticBuilder : public StreamingDiagnostic {
 public:
   /// Copy constructor.  When copied, this "takes" the diagnostic info from the
   /// input and neuters it.
-  DiagnosticBuilder(const DiagnosticBuilder &D) {
+  DiagnosticBuilder(const DiagnosticBuilder &D) : StreamingDiagnostic() {
     DiagObj = D.DiagObj;
     DiagStorage = D.DiagStorage;
     IsActive = D.IsActive;

clang/include/clang/Basic/PartialDiagnostic.h

@@ -49,7 +49,8 @@ class PartialDiagnostic : public StreamingDiagnostic {
   PartialDiagnostic(unsigned DiagID, DiagStorageAllocator &Allocator_)
       : StreamingDiagnostic(Allocator_), DiagID(DiagID) {}
 
-  PartialDiagnostic(const PartialDiagnostic &Other) : DiagID(Other.DiagID) {
+  PartialDiagnostic(const PartialDiagnostic &Other)
+      : StreamingDiagnostic(), DiagID(Other.DiagID) {
     Allocator = Other.Allocator;
     if (Other.DiagStorage) {
       DiagStorage = getStorage();