Add warning Wenum-conversion for C and ObjC.

Add warning Wenum-conversion for C and ObjC.

The patch enables warning with Wextra due to PR91593 and warnings with
allmodconfig kernel build. Once these issues are resolved, we could
consider promoting it to Wall.

2019-09-04  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

	PR c/78736
	* doc/invoke.texi: Document -Wenum-conversion.

c-family
	* c.opt (Wenum-conversion): New option.

c/
	* c-typeck.c (convert_for_assignment): Handle Wenum-conversion.

testsuite/
	* gcc.dg/Wenum-conversion.c: New test-case.

From-SVN: r275376