llvm-cov: Handle functions with no line number

llvm-cov: Handle functions with no line number

Functions may in an instrumented binary but not in the original source
when they're inserted by the compiler or the runtime. These functions
aren't meaningful to the user, so teach llvm-cov to skip over them
instead of crashing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204863 91177308-0d34-0410-b5e6-96231b3b80d8