Extend CompilationDatabase by a field for the output filename

In bigger projects like an Operating System, the same source code is
often compiled in slightly different ways. This could be the difference
between PIC and non-PIC code for static vs dynamic libraries, it could
also be the difference between size optimised versions of tools for
ramdisk images. At the moment, the compilation database has no way to
distinguish such cases. As first step, add a field in the JSON format
for it and process it accordingly.

Differential Revision: https://reviews.llvm.org/D27138

llvm-svn: 288436