Remove some ProcFileReader occurences

Summary:
ProcFileReader is the cause of the dependency from Host to ProcessLinux
module. Since it's interface is also obsolete (ReadIntoDataBuffer is
trivially replaceable by llvm::MemoryBuffer functions and
ProcessLineByLine is trivially implementable with StringRefs), instead
of moving it around I'm planning to obliterate it. This is the first
step, where I remove a couple of occurences in linux/Host.cpp, and
modernize some code around that.

I have introduced linux/Support.h, which holds two utility functions
now, whose resposibility is to construct the appropriate proc file names
-- the only useful feature of ProcFileReader.

I add a couple of tests for these functions, and for
Host::GetProcessInfo. It's worth noting that these are the first
host-specific unit tests in lldb.

Reviewers: zturner, eugene

Subscribers: srhines, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D30942

llvm-svn: 297843