Fix race in IOHandlerProcessSTDIO

Summary:
IOHandlerProcessSTDIO::Run() was opening the pipe for interrupt requests lazily. This was racing
with another thread executing IOHandlerProcessSTDIO::Cancel() simultaneously. I fix this by
opening the pipe in the object constructor. The pipe will be automatically closed when the object
is destroyed.

Test Plan: Tests pass on linux.

Reviewers: clayborg, ribrdb

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D10060

llvm-svn: 238423