[esan] EfficiencySanitizer base runtime library

Summary:
Adds the initial version of a runtime library for the new
EfficiencySanitizer ("esan") family of tools.  The library includes:

+ Slowpath code via callouts from the compiler instrumentation for
  each memory access.

+ Registration of atexit() to call finalization code.

+ Runtime option flags controlled by the environment variable
  ESAN_OPTIONS.  The common sanitizer flags are supported such as
  verbosity and log_path.

+ An initial simple test.

Still TODO: common code for libc interceptors and shadow memory mapping,
and tool-specific code for shadow state updating.

Reviewers: eugenis, vitalybuka, aizatsky, filcab

Subscribers: filcab, vkalintiris, kubabrecka, llvm-commits, zhaoqin, kcc

Differential Revision: http://reviews.llvm.org/D19168

llvm-svn: 267060