Add support to generate Clang JSON compilation database

Lot of clang based tools need knowledge of the exact compilation command line. These tools can read the command line from a clang JSON compilation database.
Cmake support this, and as a workaround for qbs, it is possible to generate it via Build EAR.

It would be nice if qbs could support this natively, as using the wrapper approach has some drawbacks (eg, you need to build the whole code base, some issues with LD_PRELOAD, ....).
maybe something like

qbs generate -g clang-tools

Ideally, Qbs would generate this file without performing any compilation steps.

The advantage over cpp.compilerWrapper is increased flexibility and decoupled build/static analysis.