cmd/link: enable linker dead code pruning when using reflect

[jim-minter]

Follow-up from https://groups.google.com/d/topic/golang-nuts/syC4LpVf7sw

The link-time optimisation to prune unused methods is disabled when reflect.Value.Call() is used anywhere within a program.

With large codebases with many vendored libraries, it's hard to avoid including something which calls reflect.Value.Call(), e.g. anything that uses text/template.

This problem is causing serious bloat on kubernetes and openshift binaries.

One improvement that might be feasible is to enable pruning of methods of a type where:

• the set of packages which use that type is statically determinable (e.g., the type doesn't leak from the package set via an exported identifier), and
• no packages in the set reference reflect.Value.Call()

The intention is to be able to prune methods which are provably non-reachable from any reflect.Value.Call() callpoint.

Such an improvement may be complex but I believe the commensurate reduction in binary sizes for projects such as kubernetes make it highly worth considering.

[jim-minter]

@crawshaw as author of most of /src/cmd/link/internal/ld/deadcode.go, what's your take on the above?

@smarterclayton fyi