x/tools/gopls: show optimization information from the compiler

[stamblerre]

@dr2chase has recently added functionality to the compiler to produce diagnostic information about which optimizations were not performed.

gopls doesn't run the compiler, so we can't integrate this information easily, but as a starting point, we can have the user run this as a command. My thinking is that the user manually executes the command, which under the hood runs the compiler with the correct flags and publishes diagnostics with the output. The main issue is - how long should these diagnostics last? With our current architecture, the next time the user types a character, these diagnostics will automatically clear.

More information can be found on https://golang.org/cl/206658/.

[stamblerre]

This feature has been implemented as of https://golang.org/cl/243119 and will be available in gopls/v0.5.0.