Remove unnecessary template specifiers from truct constructors in tests.

As far as I can see they are unnecessary and in C++20 they lead to errors.
Removing them the tests pass with C++20 and the default version.
Example error message:
```
llvm-project/llvm/unittests/IR/PassBuilderCallbacksTest.cpp:275:30: error: expected unqualified-id before ‘)’ token
  275 |   MockAnalysisHandle<Module>() { setDefaults(); }
```

Differential Revision: https://reviews.llvm.org/D141358

llvm/unittests/IR/PassBuilderCallbacksTest.cpp

@@ -236,7 +236,7 @@ struct MockAnalysisHandle<Loop>
   MOCK_METHOD3_T(invalidate, bool(Loop &, const PreservedAnalyses &,
                                   LoopAnalysisManager::Invalidator &));
 
-  MockAnalysisHandle<Loop>() { this->setDefaults(); }
+  MockAnalysisHandle() { this->setDefaults(); }
 };
 
 template <>
@@ -247,7 +247,7 @@ struct MockAnalysisHandle<Function>
   MOCK_METHOD3(invalidate, bool(Function &, const PreservedAnalyses &,
                                 FunctionAnalysisManager::Invalidator &));
 
-  MockAnalysisHandle<Function>() { setDefaults(); }
+  MockAnalysisHandle() { setDefaults(); }
 };
 
 template <>
@@ -261,7 +261,7 @@ struct MockAnalysisHandle<LazyCallGraph::SCC>
   MOCK_METHOD3(invalidate, bool(LazyCallGraph::SCC &, const PreservedAnalyses &,
                                 CGSCCAnalysisManager::Invalidator &));
 
-  MockAnalysisHandle<LazyCallGraph::SCC>() { setDefaults(); }
+  MockAnalysisHandle() { setDefaults(); }
 };
 
 template <>
@@ -272,7 +272,7 @@ struct MockAnalysisHandle<Module>
   MOCK_METHOD3(invalidate, bool(Module &, const PreservedAnalyses &,
                                 ModuleAnalysisManager::Invalidator &));
 
-  MockAnalysisHandle<Module>() { setDefaults(); }
+  MockAnalysisHandle() { setDefaults(); }
 };
 
 static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {