toke.c: Avoid extra sv_setpv for foo <newline> =>

When parsing something like

    time
       =>

if there is a global override, the parser transforms ‘time’ into
‘CORE::GLOBAL::time’ before it looks at the next line to see if there
is a fat arrow.

If it finds a fat arrow, it has to set the name back to ‘time’.

After finding a fat arrow on the line following a bareword, it was
setting the name to what appears in the program source, even when
there was no global override.  We can skip that most of the time.

All that I said about global overrides applies to ‘our’ subs, too.
‘foo’ gets transformed into ‘ThatPackage::foo’ and needs to be changed
back.  I added a test, to make sure that is not accidentally broken.

I took the liberty of changing ((SVOP*)pl_yylval.opval)->op_sv to sv
at the same time, to make the code more readable.