[asan] Support running without /proc.

Summary:
This patch lets ASan run when /proc is not accessible (ex. not mounted
yet). It includes a special test-only flag that emulates this condition
in an unpriviledged process.

This only matters on Linux, where /proc is necessary to enumerate
virtual memory mappings.

Reviewers: pcc, vitalybuka

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D55874

llvm-svn: 350101