spec: clarify what is considered a function call for len/cap special case
gccgo considers built-in function calls returning a constant not as function call (issue 7386)
go/types considers any call (regular or built-in) as a function call
The wording and examples clarify that only "function calls" that are issued
at run-time (and thus do not result in a constant result) are considered
function calls in this case.
gc is inconsistent (issue 7385)
gccgo already interprets the spec accordingly and issue 7386 is moot.
go/types considers all calls (constant or not) as function calls (issue 7457).
Fixes #7387.
Fixes #7386.
LGTM=r, rsc, iant
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/
66860046