Add Pipe::WriteWithTimeout method

Summary:
This commit adds a WriteWithTimeout method to time Pipe class, analogous to the existing
ReadWithTimeout(). It also changes the meaning of passing zero as a timeout value. Previously,
zero was used as an infinite timeout value. Now, the meaning of zero timeout to return the data
avaiable without sleeping (basically, a non-blocking operation). This makes the behaviour of Pipe
consistent with the Communication/Connection classes. For blocking operatios with infinite
timeout, I introduce a special constant for this purpose.

Reviewers: ovyalov, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11358

llvm-svn: 242764