cmd/compile: update the export version for generics
Bump the export version to a new value iexportVersionGo1_18 (2). This
will give a better error message when old compilers/tools encounter the
new export format (that includes parameterized types and functions).
We are also making a breaking change in the format:
- a 'kind' byte is added to constant values
Also updated tinter() to pass the implicit bit through during type
substitution.
Tested that all tests still pass if the iexportVersionCurrent is changed
back to 1 in typecheck/iexport.go, iimporter/iimport.go, and
gcimporter/iimport.go
Updates #47654
Change-Id: I1dbeb167a97f6c7e0b7e0c011d6bada5db312b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/357049
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Dan Scales <danscales@google.com>