Auto merge of #43008 - zackmdavis:field_init_shorthand_in_librustc, r=estebank

use field init shorthand in src/librustc/

Commentary on #37340 [suggested](https://github.com/rust-lang/rust/issues/37340#issuecomment-255513390) using the new field init syntax in the compiler. Do we care about this? If so, here's a pull request for the librustc/ directory. While [`rustfmt` might do this in the future](https://github.com/rust-lang/rust/issues/37340#issuecomment-255513712), in the meantime, some simple Python will do:

```python
#!/usr/bin/env python3

import os, re, sys

OPPORTUNITY = re.compile(r" (\w+): \1,?\n")

def field_init_shorthand_substitution(filename):
    with open(filename) as f:
        text = f.read()
        revised = OPPORTUNITY.sub(r" \1,\n", text)
    with open(filename, 'w') as f:
        f.write(revised)

def substitute_in_directory(path):
    for dirname, _subdirs, basenames in os.walk(path):
        for basename in basenames:
            field_init_shorthand_substitution(os.path.join(dirname, basename))

if __name__ == "__main__":
    substitute_in_directory(sys.argv[1])
```

**Update 3 July**: edited the search (respectively replace) regex to ` (\w+): \1,?\n` (` \1,\n`) from ` (\w+): \1,` (` \1,`)