[Support] Support both Windows and Posix paths on both platforms.

Previously which path syntax we supported dependend on what
platform we were compiling LLVM on.  While this is normally
desirable, there are situations where we need to be able to
handle a path that we know was generated on a remote host.
Remote debugging, for example, or parsing debug info.

99% of the code in LLVM for handling paths was platform
agnostic and literally just a few branches were gated behind
pre-processor checks, so this changes those sites to use
runtime checks instead, and adds a flag to every path
API that allows one to override the host native syntax.

Differential Revision: https://reviews.llvm.org/D30858

llvm-svn: 298004