[check-iwyu] Do not fail right away if IWYU_TOOL fails

Since only newer versions of IWYU_TOOL fail with an exit code, we have
to disable -e around the IWYU_TOOL invocation. Otherwise we would not
print IWYU_LOG.

tools/check-iwyu

@@ -64,10 +64,23 @@ else
 	echo "WARNING: ${IWYU_TOOL} does not support --load"
 fi
 
+set +e
 ${IWYU_TOOL} ${IWYU_TOOL_ARGS[@]} \
 			 -- \
 			 -Xiwyu --mapping_file="${MESON_SOURCE_ROOT}/iwyu-mappings.imp" \
 			 > "${IWYU_LOG}"
+set -e
+# Only newer versions of IWYU_TOOL fail with an exit code (IWYU >
+# 0.15, since commit
+# https://github.com/include-what-you-use/include-what-you-use/commit/a7499e4a2b416592777cc4c33fca746d091af738). Therefore
+# we have to manually see if this IWYU_TOOL run failed with an error
+# exit code.
+# shellcheck: disable=SC2181
+if [[ ${?} -gt 0 ]]; then
+		echoerr "IWYU exited with ${?}"
+		cat "${IWYU_LOG}"
+		exit $?
+fi
 
 if [[ ! -s "${IWYU_LOG}" ]]; then
 	echoerr "${IWYU_LOG} is empty (or non existent)"