This Go frontend patch treats S("") as a string constant.  The
compiler neglected to notice that a conversion from a string constant
to a string type was a valid string constant.

I didn't add a test case because this only caused a compiler failure
when compiling without optimization, which is not the normal case, and
is not a case that we test.

This fixes https://go.dev/issue/56113.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian