Enable C++ branch coverage when using gcov 8 or later.

Branch coverage was disabled due to a gcov bug, but this was fixed for
GCC 8: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84879.

It doesn't make sense to disable all C++ branch coverage for
everyone because of a bug in an old GCC version.

RELNOTES: Enable C++ branch coverage if gcov version is 8 or newer.

tools/test/collect_cc_coverage.sh

@@ -132,6 +132,16 @@ function gcov_coverage() {
               mkdir -p "${COVERAGE_DIR}/$(dirname ${gcno_path})"
               cp "$ROOT/${gcno_path}" "${COVERAGE_DIR}/${gcno_path}"
           fi
+
+          # Extract gcov's version: the output of `gcov --version` contains the
+          # version as a set of major-minor-patch numbers, of which we extract
+          # the major version.
+          # gcov --version outputs a line like:
+          #   gcov (Debian 7.3.0-5) 7.3.0
+          # llvm-cov gcov --version outputs a line like:
+          #   LLVM version 9.0.1
+          gcov_major_version=$("${GCOV}" --version | sed -n -E -e 's/^.*\s([0-9]+)\.[0-9]+\.[0-9]+\s?.*$/\1/p')
+
           # Invoke gcov to generate a code coverage report with the flags:
           # -i              Output gcov file in an intermediate text format.
           #                 The output is a single .gcov file per .gcda file.
@@ -148,7 +158,15 @@ function gcov_coverage() {
           # Don't generate branch coverage (-b) because of a gcov issue that
           # segfaults when both -i and -b are used (see
           # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84879).
-          "${GCOV}" -i $COVERAGE_GCOV_OPTIONS -o "$(dirname ${gcda})" "${gcda}"
+
+          # Don't generate branch coverage (-b) when using gcov 7 or earlier
+          # because of a gcov issue that segfaults when both -i and -b are used
+          # (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84879).
+          if [[ $gcov_major_version -le 7 ]]; then
+              "${GCOV}" -i $COVERAGE_GCOV_OPTIONS -o "$(dirname ${gcda})" "${gcda}"
+          else
+              "${GCOV}" -i -b $COVERAGE_GCOV_OPTIONS -o "$(dirname ${gcda})" "${gcda}"
+          fi
 
           # Check the type of output: gcov 9 or later outputs compressed JSON
           # files, but earlier versions of gcov, and all versions of llvm-cov,