rustdoc: Fix documenting rustc-macro crates

[alexcrichton]

This commit adds a "hack" to the session to track whether we're a rustdoc
session or not. If we're rustdoc then we skip the expansion to add the
rustc-macro infrastructure.

Closes #36820

[rust-highfive]

r? @Aatch

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

[alexcrichton]

r? @nrc

[Mark-Simulacrum]

Travis failed:

error: attempted access of field `actually_rustdoc` on type `&rustc::session::Session`, but no field with that name was found
   --> src/librustc_driver/driver.rs:709:9
    |
709 |     if !sess.actually_rustdoc {
    |         ^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

[alexcrichton]

Fixed!

[nrc]

@bors: r+

[bors]

📌 Commit 7724a04 has been approved by nrc

[bors]

⌛ Testing commit 7724a04 with merge e443c1b...

[alexcrichton]

@bors: retry force clean

• restarted buildbot

[bors]

⌛ Testing commit 7724a04 with merge ff71346...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: nrc
Pushing ff71346 to master...