[esan|cfrag] Create the skeleton of cfrag variable for the runtime

Summary:
Creates a global variable containing preliminary information
for the cache-fragmentation tool runtime.

Passes a pointer to the variable (null if no variable is created) to the
compilation unit init and exit routines in the runtime.

Reviewers: aizatsky, bruening

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

Differential Revision: http://reviews.llvm.org/D20541

llvm-svn: 271298