[Support] Add file lock/unlock functions

This is recommit of f51bc4fb, reverted in 8577595e, because
the function `flock` is not available on Solaris. In this variant
`flock` was replaced with `fcntl`, which is a POSIX function.

New functions `lockFile`, `tryLockFile` and `unlockFile` implement
simple file locking. They lock or unlock entire file. This must be
enough to support simulataneous writes to log files in parallel builds.

Differential Revision: https://reviews.llvm.org/D78896