Resubmit "[Tooling] Parse compilation database command lines on Windows."

This patch introduced the ability to decide at runtime whether to parse
JSON compilation database command lines using Gnu syntax or Windows
syntax.  However, there were many existing unit tests written that
hardcoded Gnu-specific paths.  These tests were now failing because
the auto-detection logic was choosing to parse them using Windows
rules.

This resubmission of the patch fixes this by introducing an enum
which defines the syntax mode, which defaults to auto-detect, but
for which the unit tests force Gnu style parsing.

Reviewed By: alexfh
Differential Revision: https://reviews.llvm.org/D23628

llvm-svn: 279120