Only check --crate-type flags if present.

Before, normal compilation and the --crate-file-name flag would
generate output based on both #![crate_type] attributes and
--crate-type flags. Now, if one or more flag is specified by command
line, only those will be used.

Closes #11573.
auto merge of #13489 : JustAPerson/rust/crate-file-name, r=alexcrichton

Before, the `--crate-file-name` flag only checked crate attributes for
possible crate types. Now, if any type is specified by one or more
`--crate-type` flags, only the filenames for those types will be
emitted, and any types specified by crate attributes will be ignored.
Support lifetime suggestion for method

This includes a change to the way lifetime names are generated. Say we
figure that `[#0, 'a, 'b]` have to be the same lifetimes, then instead
of just generating a new lifetime `'c` like before to replace them, we
would reuse `'a`. This is done so that when the lifetime name comes
from an impl, we don't give something that's completely off, and we
don't have to do much work to figure out where the name came from. For
example, for the following code snippet:

```rust
struct Baz<'x> {
    bar: &'x int
}

impl<'x> Baz<'x> {
    fn baz1(&self) -> &int {
        self.bar
    }
}
```

`[#1, 'x]` (where `#1` is BrAnon(1) and refers to lifetime of `&int`)
have to be marked the same lifetime. With the old method, we would
generate a new lifetime `'a` and suggest `fn baz1(&self) -> &'a int`
or `fn baz1<'a>(&self) -> &'a int`, both of which are wrong.