[esan|cfrag] Disable load/store instrumentation for cfrag

Summary:
Adds ClInstrumentFastpath option to control fastpath instrumentation.

Avoids the load/store instrumentation for the cache fragmentation tool.

Renames cache_frag_basic.ll to working_set_slow.ll for slowpath
instrumentation test.

Adds the __esan_init check in struct_field_count_basic.ll.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D21079

llvm-svn: 272355