This reverts commit 37a5b51.

This disables incremental (i.e., -Cincremental) taking effect unless an
environment variable, RUSTC_FORCE_INCREMENTAL, is set to 1 in the environment of
the running rustc. Currently incremental causes errors for many users, and we do
not have an expectation of being able to quickly fix these errors in a
backportable way - so, for now, disable incremental entirely. The user can still
opt-in, but this way the majority of users merely get slower builds, not broken
builds.

This won't enable incremental if -Cincremental is not already passed, but is
seemingly necessary in the presence of -Zincremental-verify-ich which does not
check for incremental being enabled before accessing incremental data
structures, leading to an ICE.

This reverts commit 448d076 to address
issue 85713 on beta.