[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: vitalybuka, pcc, krytarowski

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D56141

llvm-svn: 350590