[clang][CodeGen] Keep processing the rest of AST after encountering unsupported MC/DC expressions (#82464)

Currently, upon seeing unsupported decisions (more than 6 conditions, or
split nesting), the post-visitor hook dataTraverseStmtPost() returns a
false. As a result, in the rest of tree even supported decisions will
be skipped as well. Like in the below code:

{ // CompoundStmt
  a && b;           // 1: BinaryOperator (supported)
  a && foo(b && c); // 2: BinaryOperator (not yet supported due to split
                    //                    nesting)
  a && b;           // 3: BinaryOperator (supported)
}

Decision 3 will not be processed at all. And only one "Decision" region
will be emitted. Compiler explorer example:
https://godbolt.org/z/Px61sesoo

We hope to process such cases and emit two "Decision" regions (1 and 3)
in the above example.

Author: whentojump

clang/lib/CodeGen/CodeGenPGO.cpp

@@ -239,9 +239,12 @@ struct MapRegionCounters : public RecursiveASTVisitor<MapRegionCounters> {
     if (MCDCMaxCond == 0)
       return true;
 
-    /// At the top of the logical operator nest, reset the number of conditions.
-    if (LogOpStack.empty())
+    /// At the top of the logical operator nest, reset the number of conditions,
+    /// also forget previously seen split nesting cases.
+    if (LogOpStack.empty()) {
       NumCond = 0;
+      SplitNestedLogicalOp = false;
+    }
 
     if (const Expr *E = dyn_cast<Expr>(S)) {
       const BinaryOperator *BinOp = dyn_cast<BinaryOperator>(E->IgnoreParens());
@@ -292,7 +295,7 @@ struct MapRegionCounters : public RecursiveASTVisitor<MapRegionCounters> {
                 "contains an operation with a nested boolean expression. "
                 "Expression will not be covered");
             Diag.Report(S->getBeginLoc(), DiagID);
-            return false;
+            return true;
           }
 
           /// Was the maximum number of conditions encountered?
@@ -303,7 +306,7 @@ struct MapRegionCounters : public RecursiveASTVisitor<MapRegionCounters> {
                 "number of conditions (%0) exceeds max (%1). "
                 "Expression will not be covered");
             Diag.Report(S->getBeginLoc(), DiagID) << NumCond << MCDCMaxCond;
-            return false;
+            return true;
           }
 
           // Otherwise, allocate the number of bytes required for the bitmap