[lldb/Target] Rework the way the inferior environment is created

Summary:
The interactions between the environment settings (`target.env-vars`,
`target.inherit-env`) and the inferior life-cycle are non-obvious
today. For example, if `target.inherit-env` is set, the `target.env-vars`
setting will be augmented with the contents of the host environment
the first time the launch environment is queried (usually at
launch). After that point, toggling `target.inherit-env` will have no
effect as there's no tracking of what comes from the host and what is
a user setting.

This patch computes the environment every time it is queried rather
than updating the contents of the `target.env-vars` property. This
means that toggling the `target.inherit-env` property later will now
have the intended effect.

This patch also adds a `target.unset-env-vars` settings that one can
use to remove variables from the launch environment. Using this, you
can inherit all but a few of the host environment.

The way the launch environment is constructed is:
  1/ if `target.inherit-env` is set, then read the host environment
  into the launch environment.
  2/ Remove for the environment the variables listed in
  `target.unset-env`.
  3/ Augment the launch environment with the contents of
  `target.env-vars`. This overrides any common values with the host
  environment.

The one functional difference here that could be seen as a regression
is that `target.env-vars` will not contain the inferior environment
after launch. The patch implements a better alternative in the
`target show-launch-environment` command which will return the
environment computed through the above rules.

Reviewers: labath, jingham

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76470

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