Flags are clang's default UI.
We can have an env var in addition to that, but in D69825 nobody has yet mentioned why this needs an env var, so omit it for now.
If someone needs to set the flag via env var, the existing CCC_OVERRIDE_OPTIONS mechanism works
for it (set CCC_OVERRIDE_OPTIONS=+-fno-integrated-cc1 for example).
Also mention the cc1-in-process change in the release notes.
Also spruce up the test a bit so it actually tests something :)