[clang][deps] Make clang-scan-deps write modules in raw format

We have no use for debug info for the scanner modules, and writing raw
ast files speeds up scanning ~15% in some cases. Note that the compile
commands produced by the scanner will still build the obj format (if
requested), and the scanner can *read* obj format pcms, e.g. from a PCH.

rdar://108807592

Differential Revision: https://reviews.llvm.org/D149693