[flang] Add -O flag to tco (#151869)
At the moment, there is no established way to emit LLVM IR before
optimization in LLVM. tco can partially does, but the optimization level
is fixed to 2. This patch adds -O flag to tco.
Note that this is not completely equivalent to the frontend option. tco
does not accept -O flag without numbers, and the default optimization
level is not 0 but 2.
