[esan] Use ModulePass for EfficiencySanitizerPass.

Summary:
Uses ModulePass instead of FunctionPass for EfficiencySanitizerPass to
better support global variable creation for a forthcoming struct field
counter tool.

Patch by Qin Zhao.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D20458

llvm-svn: 270263