compiler: optimize string concatenations

compiler: optimize string concatenations

    runtime.concatstring{2,3,4,5} are just wrappers of concatstrings.
    These wrappers don't provide any benefit, at least in the C
    calling convention we use, where passing arrays by value isn't an
    efficient thing. Change it to always use concatstrings.

    Also, the cap field of the slice passed to concatstrings is not
    necessary. So change it to pass a pointer and a length directly,
    which is more efficient than passing a slice header by value.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182539

From-SVN: r272476