Introduce two command-line flags for the instrumentation pass to control whether the labels of pointers should be ignored in load and store instructions

The new command line flags are -dfsan-ignore-pointer-label-on-store and -dfsan-ignore-pointer-label-on-load. Their default value matches the current labelling scheme.

Additionally, the function __dfsan_union_load is marked as readonly.

Patch by Lorenzo Martignoni!

Differential Revision: http://llvm-reviews.chandlerc.com/D2187

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195382 91177308-0d34-0410-b5e6-96231b3b80d8