Generic function symbols sometimes have % in them, like:

    main.B2[%2eshape.string_0].m2·f

Which confuses this code because it doesn't esacpe % when
using this string as a format string, instead of a format argument.

Or could we get rid of the . -> %2e rewrite somehow?
I think it comes from LinkString.

Change-Id: I3275501f44cf30485e9d4577e0dfa77996d4939e
Reviewed-on: https://go-review.googlesource.com/c/go/+/357837


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>