Restore union-like behavior for inference to conditional types

[ahejlsberg]

Fixes #31766.

[weswigham]

Should check that that node8 test is passing, too - I restarted it because the failure seemed unrelated (maybe due to a nested dependency) hopefully it's ok on rerun.

[weswigham]

cc @rbuckton re: the node 8 failure on this PR. That failure looks like it's in an untranspiled optional catch clause in a library you happen to maintain: https://github.com/esfx/esfx/blob/master/internal/deprecate/src/index.ts

[rbuckton]

Should be fixed now. I restarted the node 8 build and it passed successfully.