Switches the JSONCompilationDatabase to use the YAML parser.

Switches the JSONCompilationDatabase to use the YAML parser.
This will allow us to delete the JSON parser from llvm.

The biggest change is a general change of strategy - instead
of storing StringRef's to the values for the command line and
directory in the input buffer, we store ScalarNode*'s. The
reason is that the YAML parser's getRawValue on ScalarNodes
returns a string that includes the quotes in case of double
quoted strings.

For the same reason we're removing the JSON parsing part of
the command line parsing - this means an extra copy for a
command line when it is requested (and only when it is requested).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154929 91177308-0d34-0410-b5e6-96231b3b80d8