[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

Change-Id: Id848e3ba7e757801fc3f93c63826d8078a755f2b
git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@350590 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed