[lldb][TypeSystemClang] Initialize ClassTemplateSpecializationDecl's StrictPackMatch field (#126215)

This addresses the MSAN failure reported
in
llvm/llvm-project#125791 (comment):
```
==5633==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 in clang::ASTNodeImporter::CallOverloadedCreateFun<clang::ClassTemplateSpecializationDecl>::operator()
    #1 in bool clang::ASTNodeImporter::GetImportedOrCreateSpecialDecl<...>
...
```

The ASTImporter reads `D->hasStrictPackMatch()` and forwards it to the
constructor of the destination `ClassTemplateSpecializationDecl`. But if
`D` is a decl that LLDB created from debug-info, it would've been
created using `ClassTemplateSpecializationDecl::CreateDeserialized`,
which doesn't initialize the `StrictPackMatch` field.

This patch just initializes the field to a fixed value of `false`, to
preserve previous behaviour and avoid the use-of-uninitialized-value.

An alternative would be to always initialize it in the
`ClassTemplateSpecializationDecl` constructor, but there were
reservations about providing a default value for it because it might
lead to hard-to-diagnose problems down the line.

Author: Michael137

clang/include/clang/AST/DeclTemplate.h

@@ -1960,6 +1960,8 @@ class ClassTemplateSpecializationDecl : public CXXRecordDecl,
 
   bool hasStrictPackMatch() const { return StrictPackMatch; }
 
+  void setStrictPackMatch(bool Val) { StrictPackMatch = Val; }
+
   /// Get the point of instantiation (if any), or null if none.
   SourceLocation getPointOfInstantiation() const {
     return PointOfInstantiation;

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -1666,6 +1666,12 @@ TypeSystemClang::CreateClassTemplateSpecializationDecl(
   ast.getTypeDeclType(class_template_specialization_decl, nullptr);
   class_template_specialization_decl->setDeclName(
       class_template_decl->getDeclName());
+
+  // FIXME: set to fixed value for now so it's not uninitialized.
+  // One way to determine StrictPackMatch would be
+  // Sema::CheckTemplateTemplateArgument.
+  class_template_specialization_decl->setStrictPackMatch(false);
+
   SetOwningModule(class_template_specialization_decl, owning_module);
   decl_ctx->addDecl(class_template_specialization_decl);