Fix detection of substituted tpl param in alias

It turns out that the presence of `SubstTemplateTypeParmType` inside
an alias type sugar chain doesn't necessarily mean that it is
a user-provided template parameter. It may occur e. g. when an alias
template is used inside the `typedef` declaration, and its template
parameter is specified inside that declaration. The regression had been
introduced in 2d26dd1 and found on MS STL `vector` implementation
when IWYU started to require `_Simple_types` type info for such a code:

std::vector v{1, 2, 3};
(void)v.begin();

due to the returned `iterator` type alias.

iwyu_ast_util.cc

@@ -568,6 +568,11 @@ static bool IsSubstTemplateTypeParmType(const Type* type) {
   do {
     if (isa<SubstTemplateTypeParmType>(type))
       return true;
+    if (isa<TypedefType>(type) || isa<TemplateSpecializationType>(type)) {
+      // Still may be substituted outer template parameter but needs additional
+      // analysis outside this function.
+      return false;
+    }
     prev_type = type;
     type = type->getLocallyUnqualifiedSingleStepDesugaredType().getTypePtr();
   } while (type != prev_type);

tests/cxx/typedef_chain_no_follow-d4.h

@@ -0,0 +1,15 @@
+//===--- typedef_chain_no_follow-d4.h - test input file for iwyu ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "tests/cxx/typedef_chain_class.h"
+
+template <typename T>
+using IdentityAlias = T;
+
+using ProvidingWithAliasTpl = IdentityAlias<TypedefChainClass>;

tests/cxx/typedef_chain_no_follow.cc

@@ -15,13 +15,17 @@
 #include "tests/cxx/typedef_chain_no_follow-d1.h"
 #include "tests/cxx/typedef_chain_no_follow-d2.h"
 #include "tests/cxx/typedef_chain_no_follow-d3.h"
+#include "tests/cxx/typedef_chain_no_follow-d4.h"
 // Unused include to trigger IWYU summary telling what symbols are used from
 // every file.
 #include "tests/cxx/direct.h"
 
 void TypedefDeclaredInGlobalNamespace() {
   TypedefChainTypedef tct;
   tct.Method();
+
+  ProvidingWithAliasTpl pwat;
+  pwat.Method();
 }
 
 // Tests how we handle a typedef declared in a class.  Main purpose is to make
@@ -50,5 +54,6 @@ The full include-list for tests/cxx/typedef_chain_no_follow.cc:
 #include "tests/cxx/typedef_chain_no_follow-d1.h"  // for TypedefChainTypedef
 #include "tests/cxx/typedef_chain_no_follow-d2.h"  // for NonContainer1
 #include "tests/cxx/typedef_chain_no_follow-d3.h"  // for NonContainer2
+#include "tests/cxx/typedef_chain_no_follow-d4.h"  // for ProvidingWithAliasTpl
 
 ***** IWYU_SUMMARY */