[asan] Set debug location in ASan function prologue.

Most importantly, it gives debug location info to the coverage callback.

This change also removes 2 cases of unnecessary setDebugLoc when IRBuilder
is created with the same debug location.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208767 91177308-0d34-0410-b5e6-96231b3b80d8