rustc should accept rustc -O -C opt-level=x

[bluss]

rustc -O -O is ok just like rustc -C opt-level=1 -C opt-level=2. -O is documented to be the same as -C opt-level=2, so there should be no conflict, just have the last argument take precedent.

-g vs -C debuginfo=2 behave the same way.

$ rustc -O -C opt-level=2
error: -O and -C opt-level both provided

$ rustc -C opt-level=1 -C opt-level=2 -g -g foo.rs
# ok.

[steveklabnik]

Triage: no change