[asan] Add a flag to control asm instrumentation.

With this change, asm instrumentation is disabled by default.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208167 91177308-0d34-0410-b5e6-96231b3b80d8
Work-around MSVS build breakage due to r208148

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208171 91177308-0d34-0410-b5e6-96231b3b80d8