cmd/compile: allow spaces in types.(*Type).LinkString()

Go back to allowing spaces in types.(*Type).LinkSring().

Delve folks prefer that there are spaces in type names, if needed, since
DWARF expects type names to be "a string representing the name as it
appears in the source program". At this point, it doesn't necessarily
seem worth having a separate function that removes spaces, only for use
when printing the type names in a function/method instantiation.

Most names of function/method instantiations will still not have spaces,
since they most type args are named or builtin types. I confirmed that
we are back to the original definition of LinkString(), except for some
comment changes.

Change-Id: Iab7143f659ddea4f099c6230cb134edf8aaef868
Reviewed-on: https://go-review.googlesource.com/c/go/+/355354


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