[dev.link] cmd/internal/dwarf: expand import path in function DIE
Currently, at compile time we emit a function DIE with '"".' in
the function name, and we expand it at link time, with a really
ugly function. We can just expand it at compile time instead.
This way, we don't need to modify the symbol content at link time,
and also no need to allocate memory for that.
Keep the linker expansion, in case the compiler is invoked
without the import path.
Change-Id: Id53cd2e2d3eb61efceb8d44479c4b6ef890baa43
Reviewed-on: https://go-review.googlesource.com/c/go/+/204826 Reviewed-by: Than McIntosh <thanm@google.com>