[dev.typeparams] cmd/compile: remove special escape analysis tags

This CL removes the special escape analysis tags added to support
//go:uintptrescapes and calls to external functions. Instead, these
are kept as function pragmas.

This CL by itself isn't very interesting, but I expect will help with
subsequent cleanups I have planned here.

Change-Id: Ifb960289a27e0a6295ce2d2f5ec233cac590522b
Reviewed-on: https://go-review.googlesource.com/c/go/+/329969


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>