[yaml2obj] - Add a way to set default values for macros used in a YAML.

Currently we have to override all macros that are declared. But in many
cases it is convenient to use default values and to override only
a particular one or two.

This provides a way to set a default value for any macro:

```
Symbols:
  - Name: [[FOO=foo]]
```

Differential revision: https://reviews.llvm.org/D82455