llvm-cov: Replaced asserts with proper error handling.

Unified the interface for read functions. They all return a boolean
indicating if the read from file succeeded. Functions that previously
returned the read value now store it into a variable that is passed in
by reference instead. Callers will need to check the return value to
detect if an error occurred.

Also added a new test which ensures that no assertions occur when file
contains invalid data. llvm-cov should return with error code 1 upon
failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194635 91177308-0d34-0410-b5e6-96231b3b80d8