[Coverage][NFC] Avoid visiting non-unique OpaqueValueExpr (llvm#88910)

Only unique `OpaqueValueExpr`s should be handled in the mapping builder,
as
[discussed](llvm#85837 (comment))
in llvm#85837. However, `getCond()` returns non-unique `OpaqueValueExpr` for
`BinaryConditionalOperator` (because it is also used as the "true"
branch expression). Use `getCommon()` instead so as to bypass the
`OpaqueValueExpr`.

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -2011,11 +2011,13 @@ struct CounterCoverageMappingBuilder
     Counter TrueCount = llvm::EnableSingleByteCoverage
                             ? getRegionCounter(E->getTrueExpr())
                             : getRegionCounter(E);
-
-    propagateCounts(ParentCount, E->getCond());
     Counter OutCount;
 
-    if (!isa<BinaryConditionalOperator>(E)) {
+    if (const auto *BCO = dyn_cast<BinaryConditionalOperator>(E)) {
+      propagateCounts(ParentCount, BCO->getCommon());
+      OutCount = TrueCount;
+    } else {
+      propagateCounts(ParentCount, E->getCond());
       // The 'then' count applies to the area immediately after the condition.
       auto Gap =
           findGapAreaBetween(E->getQuestionLoc(), getStart(E->getTrueExpr()));
@@ -2024,8 +2026,6 @@ struct CounterCoverageMappingBuilder
 
       extendRegion(E->getTrueExpr());
       OutCount = propagateCounts(TrueCount, E->getTrueExpr());
-    } else {
-      OutCount = TrueCount;
     }
 
     extendRegion(E->getFalseExpr());