Add support for generating Compdb file

Some tools (i.e. you-complete-me) make use of a 'compdb' file
(compile_commands.json) that records (among other things) the
arguments needed to compile a file. These tools can use this with
libclang to provide semantic completions and perform other IDE
actions. This CL adds support for soong to generate a (simple)
compile_commands.json file.

Test: make SOONG_GEN_COMPDB=1 SOONG_LINK_COMPDB_TO=$ANDROID_BUILD_TOP nothing
      examine $ANDROID_BUILD_TOP/compile_commands.json

Change-Id: I751bb344b90dfcdad1dfd71c2a85bacd345f0464