[asan] don't instrument functions with available_externally linkage. This saves a bit of compile time and reduces the number of redundant global strings generated by asan (https://code.google.com/p/address-sanitizer/issues/detail?id=167) 

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177250 91177308-0d34-0410-b5e6-96231b3b80d8