[DWARF parser] Turn DILineInfo into a struct.

Immutable DILineInfo doesn't bring any benefits and complicates
code. Also, use std::string instead of SmallString<16> for file
and function names - their length can vary significantly.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206654 91177308-0d34-0410-b5e6-96231b3b80d8