This patch attempts to primitive support for Win64 asan
Some known issues are:
When "head" include instructions that involve branching, the "cut and paste" approach may break down in a way that function interception still work but calling back the original function does not work.
The jmp [rip -8] saves some bytes in the "head" but finding the safe zone of 0xCC is not implemented yet. So it may stomp on preceding codes.
The shadow offset is not working yet on Win64. More complexity maybe involved since there are some differences regarding virtual address space between Window 8 and Windows 8.1/10.
Patch by: Wang Wei
Differential Revision: http://reviews.llvm.org/D20884