compiler: error on func declaration/definition

compiler: error on func declaration/definition

    Long long long ago Go permitted writing
        func F()
    in one file and writing
        func F() {}
    in another file.  This was removed from the language, and that is now
    considered to be a multiple definition error.  Gccgo never caught up
    to that, and it has been permitting this invalid code for some time.

    Stop permitting it, so that we give correct errors.  Since we've
    supported it for a long time, the compiler uses it in a couple of
    cases: it predeclares the hash/equal methods if it decides to create
    them while compiling another function, and it predeclares main.main as
    a mechanism for getting the right warning if a program uses the wrong
    signature for main.  For simplicity, keep those existing uses.

    This required a few minor changes in libgo which were relying,
    unnecessarily, on the current behavior.

    Reviewed-on: https://go-review.googlesource.com/93083

From-SVN: r257600