[clang] Initialize DeclaratorDecl.ExtInfo.TInfo to null (#114198)

I believe this has no effect current behavior but would make code safer
in case we forget to initialize this.

Author: bricknerb

clang/include/clang/AST/Decl.h

@@ -737,7 +737,7 @@ class DeclaratorDecl : public ValueDecl {
   // qualifier, to be used for the (uncommon) case of out-of-line declarations
   // and constrained function decls.
   struct ExtInfo : public QualifierInfo {
-    TypeSourceInfo *TInfo;
+    TypeSourceInfo *TInfo = nullptr;
     Expr *TrailingRequiresClause = nullptr;
   };