[compdb] Expand response files inline based on a switch.

References to response files in a clang compile_commands.json file can
be tricky to deal with when tooling expects all the command flags to be
present in the 'command' field.

This change introduces a '-x' option to '-t compdb' that will expand
@rspfile style response file invocations inline.

E.g.

```sh
$ ninja -t compdb cc
[
  {
    "directory": "/src/foo",
    "command": "cc -foo -bar @foo.obj.rsp",
    "file": "foo.cc"
  }
]

$ ninja -t compdb -x cc
[
  {
    "directory": "/src/foo",
    "command": "cc -foo -bar foo.cc",
    "file": "foo.cc"
  }
]
```

0 files changed, 0 insertions, 0 deletions