Allow clang to write compilation database records.

When integrating compilation database output into existing build
systems, two approaches dominate so far. Ad-hoc implementation of the
JSON output rules or using compiler wrappers. This patch adds a new
option "-MJ foo.json" which gives a slightly cleaned up compilation
record. The output is a fragment, i.e. you still need to add the array
markers, but it allows multiple files to be easy merged.

This way the only change in a build system is adding the option with
potentially a per-target output file and merging the files with
something like
  (echo '['; cat *.o.json; echo ']' > compilation_database.json
or some additional filtering to remove the trailing comma for strict
JSON compliance.

Differential Revision: https://reviews.llvm.org/D27140

llvm-svn: 288821