Do not crash if IsInHeader is called for builtin

In very particular circumstances (see test case), VisitFunctionDecl
would see a function definition that was an implicit constructor of a
builtin (va_list_tag).

Rather than crashing for implicit code, just say it's not in a header.

This feels a little questionable, because depending on how IsInHeader is
used, it might be misleading that it returns false for builtins/implicit
code. But I think it makes sense for now.

Fixes issue #1162.

iwyu.cc

@@ -1777,7 +1777,7 @@ class IwyuBaseAstVisitor : public BaseAstVisitor<Derived> {
       }
       if (!IsInHeader(decl)) {
         // No point in author-intent analysis of function definitions
-        // in source files.
+        // in source files or for builtins.
         return true;
       }
     } else {

iwyu_location_util.cc

@@ -174,7 +174,11 @@ bool IsInScratchSpace(SourceLocation loc) {
 
 bool IsInHeader(const clang::Decl* decl) {
   const FileEntry* containing_file = GetFileEntry(decl);
-  CHECK_(containing_file);
+  if (!containing_file) {
+    // This is a builtin, or something is terribly wrong.
+    // At any rate, we're not in a header.
+    return false;
+  }
   return !GlobalSourceManager()->isMainFile(*containing_file);
 }
 

tests/cxx/template_varargs.cc

@@ -0,0 +1,59 @@
+//===--- template_varargs.cc - 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.
+//
+//===----------------------------------------------------------------------===//
+
+// This is a testcase heavily reduced from the most representative
+// Boost.Serialization example available online:
+// https://stackoverflow.com/a/33226687/96963.
+//
+// This input would previously crash IWYU because the implicit constructor of
+// __builtin_va_list inside the virtual member function in a template does not
+// have any location information.
+
+template <class T>
+class Varargs {
+  // Virtual member function is traversed as part of instantiation.
+  virtual void* unused(unsigned, ...) const {
+    __builtin_va_list x;
+    return nullptr;
+  }
+};
+
+template <class T>
+struct Creator {
+  static T make() {
+    return T();
+  }
+};
+
+// Necessary complications to instantiate Varargs in a deeply nested context.
+struct Base {
+  Base(const Varargs<int>&);
+};
+
+template <class T>
+struct Derived : Base {
+  Derived() : Base(Creator<Varargs<int>>::make()) {
+  }
+};
+
+template <class T>
+void InstantiateDerived() {
+  // Only instantiate the template, don't call the method.
+  (void)Creator<Derived<T>>::make;
+}
+
+void p() {
+  InstantiateDerived<int>();
+}
+
+/**** IWYU_SUMMARY
+
+(tests/cxx/template_varargs.cc has correct #includes/fwd-decls)
+
+***** IWYU_SUMMARY */