Make the driver API a little more useful for a tools shim

[nrc]

Example use case: https://github.com/nrc/rls-rustc

[rust-highfive]

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

[nrc]

r? @eddyb

[eddyb]

@bors r+

[bors]

📌 Commit 540a644 has been approved by eddyb

[arielb1]

[00:31:18] Compiling rustdoc v0.0.0 (file:///checkout/src/librustdoc)
[00:31:23] error[E0061]: this function takes 3 parameters but 2 parameters were supplied
[00:31:23] --> /checkout/src/librustdoc/core.rs:158:55
[00:31:23] |
[00:31:23] 158 | let krate = panictry!(driver::phase_1_parse_input(&sess, &input));
[00:31:23] | ^^^^^^^^^^^^^ expected 3 parameters
[00:31:23] [00:31:25] error[E0061]: this function takes 3 parameters but 2 parameters were supplied
[00:31:25] --> /checkout/src/librustdoc/test.rs:94:55
[00:31:25] |
[00:31:25] 94 | let krate = panictry!(driver::phase_1_parse_input(&sess, &input));
[00:31:25] | ^^^^^^^^^^^^^ expected 3 parameters
[00:31:25] [00:31:25] error: aborting due to 2 previous errors

@bors r-

[nrc]

@bors: r=eddyb

[bors]

📌 Commit 8ba63a1 has been approved by eddyb

[bors]

⌛ Testing commit 8ba63a1bc909d9f72f8d820a2c62b509cb12cdb3 with merge b0a47f7a9987e0285c3957c58ebb3eac62cf852f...

[bors]

💔 Test failed - status-travis

[nrc]

@bors: r=eddyb

[bors]

📌 Commit a9a181d has been approved by eddyb

[bors]

⌛ Testing commit a9a181d with merge 57e720d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 57e720d to master...