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) {
   ^