[AST] Initialized AutoTypeLocInfo::FoundDecl

Msan complains if getFoundDecl called before it's set.
This looks like data class, so I see no reason to keep
the single field uninitialized.

clang/include/clang/AST/TypeLoc.h

@@ -2102,7 +2102,7 @@ struct AutoTypeLocInfo : TypeSpecLocInfo {
   NestedNameSpecifierLoc NestedNameSpec;
   SourceLocation TemplateKWLoc;
   SourceLocation ConceptNameLoc;
-  NamedDecl *FoundDecl;
+  NamedDecl *FoundDecl = nullptr;
   SourceLocation LAngleLoc;
   SourceLocation RAngleLoc;