[driver] add a new option -gen-cdb-fragment-path to emit
a fragment of a compilation database for each compilation
This patch adds a new option called -gen-cdb-fragment-path to the driver,
which can be used to specify a directory path to which clang can emit a fragment
of a CDB for each compilation it needs to invoke.
This option emits the same CDB contents as -MJ, and will be ignored if -MJ is specified.
Differential Revision: https://reviews.llvm.org/D66555