[PATCH] Fix JSON compilation database command unescaping.

[Tomasz Miąsko]

Why limit this to double quote only? According to our specification both '"'
and '\' are special characters, thus both of them should have been escaped when
occurring in context where they have special meaning but literal meaning is
intended.

Consider for example ninja build command that executes /bin/echo with single
backslash as an argument:
```
$ cat build.ninja 
rule cc
  command = /bin/echo "\\"
build test: cc test.c
$ ninja -t compdb cc
[
  {
    "directory": "/tmp/",
    "command": "/bin/echo \"\\\\\"",
    "file": "test.c"
  }
]
```

http://reviews.llvm.org/D6376