[esan] Add compilation unit init/exit routines.

Summary:
Adds processCompilationUnitInit and processCompilationUnitExit
for compilation unit init/exit.

Adds a tool-specific argument passed to esan_init.

Adds the dtor and esan_exit called from the dtor.

A test will be added separately (adding it here results in failure until
the corresponding compilation patch is in place).

Reviewers: aizatsky

Subscribers: kubabrecka, bruening, kcc, vitalybuka, eugenis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20486

llvm-svn: 270624