[TestAttachDenied] Use a file instead of a pipe for synchronization.

Summary:
One cannot use mknod or mkfifo on user Android devices. This commit
changes the use of pipe to a file to synchronize between the inferior
and the test.

Test Plan: dotest.py -P TestAttachDenied

Reviewers: ovyalov, chaoren

Reviewed By: chaoren

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D9768

llvm-svn: 237328