[clang-tidy] Fix definitions in headers check to respect qualifiers

[kadircet]

Summary

The check was generating a fix without taking qualifiers in return type
into account. This patch changes the insertion location to be before qualifers.

[hokein]

thanks for the fix.

clang-tools-extra/clang-tidy/misc/DefinitionsInHeadersCheck.cpp

if (const auto *FD = dyn_cast<FunctionDecl>(ND)) {
     bool IsFullSpec = FD->getTemplateSpecializationKind() != TSK_Undeclared;
     diag(FD->getLocation(),
          "%select{function|full function template specialization}0 %1 defined "
          "in a header file; function definitions in header files can lead to "
          "ODR violations")
         << IsFullSpec << FD;
     diag(FD->getLocation(), /*FixDescription=*/"make as 'inline'",
          DiagnosticIDs::Note)
-        << FixItHint::CreateInsertion(FD->getReturnTypeSourceRange().getBegin(),
-                                      "inline ");
+        << FixItHint::CreateInsertion(FD->getInnerLocStart(), "inline ");
   } else if (const auto *VD = dyn_cast<VarDecl>(ND)) {
     // Static data members of a class template are allowed.
     if (VD->getDeclContext()->isDependentContext() && VD->isStaticDataMember())
       return;
     // Ignore instantiated static data members of classes.
     if (isTemplateInstantiation(VD->getTemplateSpecializationKind()))
       return;
     // Ignore variable definition within function scope.

clang-tools-extra/test/clang-tidy/misc-definitions-in-headers.hpp

 template <typename T>
 int CD<T, int>::f() { // OK: partial template specialization.
   return 0;
 }
 
 constexpr int k = 1; // OK: constexpr variable has internal linkage.
 
 constexpr int f10() { return 0; } // OK: constexpr function definition.
+
+const int f11() { return 0; }
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: function 'f11' defined in a header file;
+// CHECK-FIXES: inline const int f11() { return 0; }
+
+template <typename T>
+const T f12();
+
+template <>
+const int f12() { return 0; }
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: full function template specialization 'f12<int>' defined in a header file;
+// CHECK-FIXES: inline const int f12() { return 0; }