Breakpad: auto-detect path style of file entries

Summary:
This adds support for auto-detection of path style to SymbolFileBreakpad
(similar to how r351328 did the same for DWARF). We guess each file
entry separately, as we have no idea which file came from which compile
units (and different compile units can have different path styles). The
breakpad generates should have already converted the paths to absolute
ones, so this guess should be reasonable accurate, but as always with
these kinds of things, it is hard to give guarantees about anything.

In an attempt to bring some unity to the path guessing logic, I move the
guessing logic from inside SymbolFileDWARF into the FileSpec class and
have both symbol files use it to implent their desired behavior.

Reviewers: clayborg, lemo, JDevlieghere

Subscribers: aprantl, markmentovai, lldb-commits

Differential Revision: https://reviews.llvm.org/D57895

llvm-svn: 353702