Change type of DiagnosticHandlerTy (#86504)

Changing type of DiagnosticHandlerTy due to adding
-Wcast-function-type-mismatch to -Wextra
group(llvm/llvm-project#86131 (comment)).
Changed the reference argument DiagnosticInfo to a pointer and edited
the test cases failing due to this change. Added another small change
where Gtest api was throwing an warning due varargs argument not being
passed.

Author: Abhinkop

llvm/include/llvm/IR/DiagnosticHandler.h

@@ -27,7 +27,7 @@ struct DiagnosticHandler {
       : DiagnosticContext(DiagContext) {}
   virtual ~DiagnosticHandler() = default;
 
-  using DiagnosticHandlerTy = void (*)(const DiagnosticInfo &DI, void *Context);
+  using DiagnosticHandlerTy = void (*)(const DiagnosticInfo *DI, void *Context);
 
   /// DiagHandlerCallback is settable from the C API and base implementation
   /// of DiagnosticHandler will call it from handleDiagnostics(). Any derived
@@ -41,7 +41,7 @@ struct DiagnosticHandler {
   /// with a prefix based on the severity.
   virtual bool handleDiagnostics(const DiagnosticInfo &DI) {
     if (DiagHandlerCallback) {
-      DiagHandlerCallback(DI, DiagnosticContext);
+      DiagHandlerCallback(&DI, DiagnosticContext);
       return true;
     }
     return false;

llvm/unittests/Linker/LinkModulesTest.cpp

@@ -71,7 +71,7 @@ class LinkModuleTest : public testing::Test {
   BasicBlock *ExitBB;
 };
 
-static void expectNoDiags(const DiagnosticInfo &DI, void *C) {
+static void expectNoDiags(const DiagnosticInfo *DI, void *C) {
   llvm_unreachable("expectNoDiags called!");
 }