[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

llvm-svn: 369938