Write generics of trait aliases into metadata.

[davidtwco]

Fixes #57676.

This fixes an ICE where the generics of trait aliases are being read
from metadata when assembling the list of extension candidates and one
such candidate is a trait alias defined in another crate.

[rust-highfive]

r? @michaelwoerister

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[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:1af483e2:start=1548019910234785183,finish=1548019992659487595,duration=82424702412
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
[00:13:56]    Compiling rustc_typeck v0.0.0 (/checkout/src/librustc_typeck)
[00:13:58] error: unreachable pattern
[00:13:58]     --> src/librustc_metadata/encoder.rs:1224:17
[00:13:58]      |
[00:13:58] 1224 |                 hir::ItemKind::TraitAlias(..) => Some(self.encode_generics(def_id)),
[00:13:58]      |
[00:13:58]      = note: `-D unreachable-patterns` implied by `-D warnings`
[00:13:58] 
[00:13:58] error: aborting due to previous error
---
[00:18:32] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json"
[00:18:32] expected success, got: exit code: 101
[00:18:32] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:18:32] Build completed unsuccessfully in 0:15:03
[00:18:32] make: *** [all] Error 1
[00:18:32] Makefile:18: recipe for target 'all' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:08af59c4
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Sun Jan 20 21:51:53 UTC 2019

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[davidtwco]

Closing, looks like my local build was out-of-date and this had already been fixed in the past day or two.