gccgo: allows multiple declarations of a function

[Dmitry Vyukov]

gcc successfully compiles the following program:

package a
func n()
func n()

Both gc and go/types reject it. Compilers need to agree.

gcc version 6.0.0 20150702 (experimental) (GCC)

[Dmitry Vyukov]

@paranoiacblack

[GopherBot]

CL https://golang.org/cl/12508 mentions this issue.