[DebugLine] Improve path handling.

After switching over LLDB's line table parser to libDebugInfo, we
noticed two regressions on the Windows bot. The problem is that when
obtaining a file from the line table prologue, we append paths without
specifying a path style. This leads to incorrect results on Windows for
debug info containing Posix paths:

  0x0000000000201000: /tmp\b.c, is_start_of_statement = TRUE

This patch is an attempt to fix that by guessing the path style whenever
possible.

Differential revision: https://reviews.llvm.org/D66227

llvm-svn: 368879