[Profile] Remove duplicate file locks when enabled continuous mode and online merging.

In `initializeProfileForContinuousMode`, we have already locked the profile file when merging is enabled, so there's no need to lock the same file second time in `openFileForMerging`.

On Linux/Darwin, the locking the same file twice doesn't cause any problem. But on Windows, it causes the problem to hang forever.

With this minor fix, continuous mode seems working with online merging on Windows.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D154748