[esan] Specify which tool via a global variable

Summary:
Adds a global variable to specify the tool, to support handling early
interceptors that invoke instrumented code and require shadow memory to be
initialized prior to __esan_init() being invoked.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D20973

llvm-svn: 271715