cmd/covdata: relax mode clash policy for selected operations

Relax the policy on counter mode clashes in certain cases for "go tool
covdata" operations. Specifically, when generating 'percent',
'pkglist' or 'func' reports, we only care about whether a given
statement is executed, thus counter mode clashes are irrelevant; there
is no need to report clashes for these ops.

Example:

  $ go build -covermode=count -o myprog.count.exe myprog
  $ go build -covermode=set -o myprog.set.exe myprog
  $ GOCOVERDIR=dir1 ./myprog.count.exe
  ...
  $ GOCOVERDIR=dir2 ./myprog.set.exe
  ...
  $ go tool covdata percent i=dir1,dir2
  error: counter mode clash while reading meta-data file dir2/covmeta.1a0cd0c8ccab07d3179f0ac3dd98159a: previous file had count, new file has set
  $

With this patch the command above will "do the right thing" and work
properly, and in addition merges using the "-pcombine" flag will also
operate with relaxed rules. Note that textfmt operations still require
inputs with consistent coverage modes.

Change-Id: I01e97530d9780943c99b399d03d4cfff05aafd8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/495440
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>