Coverage: Fix iterated type for LineCoverageIterator

LineCoverageIterator is not providing access to a mutable object. Fix it
to iterate over `const LineCoverageStats` so that `operator->()`
compiles again after 6b9b86db9d.

llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h

@@ -693,8 +693,9 @@ public:
 /// An iterator over the \c LineCoverageStats objects for lines described by
 /// a \c CoverageData instance.
 class LineCoverageIterator
-    : public iterator_facade_base<
-          LineCoverageIterator, std::forward_iterator_tag, LineCoverageStats> {
+    : public iterator_facade_base<LineCoverageIterator,
+                                  std::forward_iterator_tag,
+                                  const LineCoverageStats> {
 public:
   LineCoverageIterator(const CoverageData &CD)
       : LineCoverageIterator(CD, CD.begin()->Line) {}
@@ -711,8 +712,6 @@ public:
 
   const LineCoverageStats &operator*() const { return Stats; }
 
-  LineCoverageStats &operator*() { return Stats; }
-
   LineCoverageIterator &operator++();
 
   LineCoverageIterator getEnd() const {

llvm/unittests/ProfileData/CoverageMappingTest.cpp

@@ -693,6 +693,9 @@ TEST_P(CoverageMappingTest, test_line_coverage_iterator) {
     ++Line;
   }
   ASSERT_EQ(11U, Line);
+
+  // Check that operator->() works / compiles.
+  ASSERT_EQ(1U, LineCoverageIterator(Data)->getLine());
 }
 
 TEST_P(CoverageMappingTest, uncovered_function) {