[clang][c++20] Fix code coverage mapping crash with generalized NTTPs (#85837)

Introduced in #78041, originally reported as #79957 and fixed partially
in #80050.

`OpaqueValueExpr` used with `TemplateArgument::StructuralValue` has no
corresponding source expression.

A test case with subobject-referring NTTP added.

Author: bolshakov-a

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -2271,7 +2271,8 @@ struct CounterCoverageMappingBuilder
   }
 
   void VisitOpaqueValueExpr(const OpaqueValueExpr* OVE) {
-    Visit(OVE->getSourceExpr());
+    if (OVE->isUnique())
+      Visit(OVE->getSourceExpr());
   }
 };
 

clang/test/CoverageMapping/templates.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -mllvm -emptyline-comment-coverage=false -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -main-file-name templates.cpp %s | FileCheck %s
+// RUN: %clang_cc1 -std=c++20 -mllvm -emptyline-comment-coverage=false -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -main-file-name templates.cpp %s | FileCheck %s
 
 template<typename T>
 void unused(T x) {
@@ -30,5 +30,6 @@ namespace structural_value_crash {
 
   void test() {
     tpl_fn<arr>();
+    tpl_fn<&arr[1]>();
   }
 }