compiler: add an option to emit optimization diagnostics

compiler: add an option to emit optimization diagnostics

    Add a -fgo-debug-optimization option to emit optimization
    diagnostics. This can be used for testing optimizations. Apply
    this to the range clear optimizations of maps and arrays.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/170002

gcc/go:

	* lang.opt (-fgo-debug-optimization): New option.
	* go-c.h (struct go_create_gogo_args): Add debug_optimization
	field.
	* go-lang.c (go_langhook_init): Set debug_optimization field.
	* gccgo.texi (Invoking gccgo): Document -fgo-debug-optimization.

gcc/testsuite:

	* go.dg/arrayclear.go: New test.
	* go.dg/mapclear.go: New test.

From-SVN: r270993