Add structed way to express command line options in the compilation database.

Add structed way to express command line options in the compilation database.

Currently, arguments are passed via the string attribute 'command',
assuming a shell-escaped / quoted command line to extract the original
arguments. This works well enough on Unix systems, but turns out to be
problematic for Windows tools to generate.

This CL adds a new attribute 'arguments', an array of strings, which
specifies the exact command line arguments. If 'arguments' is available
in the compilation database, it is preferred to 'commands'.

Currently there is no plan to retire 'commands': there are enough
different use cases where users want to create their own mechanism for
creating compilation databases, that it doesn't make sense to force them
all to implement shell command line parsing.

Patch by Daniel Dilts.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245036 91177308-0d34-0410-b5e6-96231b3b80d8