execgen: preserve comments around callsite of inlined function

[DrewKimball]

This patch modifies the execgen inline directive to ensure that
comments before and after the callsites of inlined functions are
not removed. Example:

func a() {
  // This comment should not be removed.
  b()
  // This one too.
}

// execgen:inline
func b() {
  foo = bar
}
=>
func a() {
  // This comment should not be removed.
  {
    foo = bar
  }
  // This one too.
}

// execgen:inline
const _ = "inlined_b"

Release note: None

[cockroach-teamcity]

This change is 

[jordanlewis]

Reviewable status: complete! 1 of 0 LGTMs obtained

[DrewKimball]

TFTR!

[DrewKimball]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)