[dev.link] move FuncID creation into the compiler/assembler

[dev.link] move FuncID creation into the compiler/assembler

Leaving creation of the funcID till the linker requires the linker to
load the function and file names into memory. Moving these into the
compiler/assembler prevents this.

This work is a step towards moving all func metadata into the compiler.

Change-Id: Iebffdc5a909adbd03ac263fde3f4c3d492fb1eac
Reviewed-on: https://go-review.googlesource.com/c/go/+/244024
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>