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

docs/compdb.md

@@ -0,0 +1,27 @@
+# Compdb (compile\_commands.json) Generator
+
+Soong can generate compdb files. This is intended for use with editing tools
+such as YouCompleteMe and other libclang based completers.
+
+compdb file generation is enabled via environment variable:
+
+```bash
+$ export SOONG_GEN_COMPDB=1
+$ export SOONG_GEN_COMPDB_DEBUG=1
+```
+
+One can make soong generate a symlink to the compdb file using an environment
+variable:
+
+```bash
+$ export SOONG_LINK_COMPDB_TO=$ANDROID_HOST_OUT
+```
+
+You can then trigger an empty build:
+
+```bash
+$ make nothing
+```
+
+Note that if you build using mm or other limited makes with these environment
+variables set the compdb will only include files in included modules.