Auto merge of #45810 - SimonSapin:ac-dc, r=aturon

Disable LLVM assertions on Nightly, enable them in "alt" builds.

Per IRC discussion https://mozilla.logbot.info/rust-infra/20171106#c13812170-c13812204

Background: https://internals.rust-lang.org/t/disabling-llvm-assertions-in-nightly-builds/5388/14

diff --combined .travis.yml
index 33982838eae32a68ce84b1b272d099d82ad65262,af418c15cdde790d59375002613ca72b3d953fe9..62336a7ff2ad58d8093111e6be8d3f6b5f388119
--- 1/.travis.yml
--- 2/.travis.yml
+++ b/.travis.yml
@@@ -16,14 -16,14 +16,14 @@@ matrix
        if: type = pull_request OR branch = auto
  
      - env: IMAGE=dist-x86_64-linux DEPLOY=1
-       if: branch = try OR branch = auto
+       if: branch = auto
  
      # "alternate" deployments, these are "nightlies" but don't have assertions
      # turned on, they're deployed to a different location primarily for projects
      # which are stuck on nightly and don't want llvm assertions in the artifacts
      # that they use.
      - env: IMAGE=dist-x86_64-linux DEPLOY_ALT=1
-       if: branch = auto
+       if: branch = try OR branch = auto
  
      - env: >
          RUST_CHECK_TARGET=dist
@@@ -253,14 -253,7 +253,14 @@@ after_failure
  
    # Random attempt at debugging currently. Just poking around in here to see if
    # anything shows up.
 -  - ls $HOME/Library/Logs/DiagnosticReports/
 +  - ls -lat $HOME/Library/Logs/DiagnosticReports/
 +  - find $HOME/Library/Logs/DiagnosticReports/ ! \(
 +      -name '*.stage2-*.crash'
 +      -name 'com.apple.CoreSimulator.CoreSimulatorService-*.crash'
 +    \)
 +      -exec echo -e travis_fold":start:crashlog\n\033[31;1m" {} "\033[0m" \;
 +      -exec head -750 {} \;
 +      -exec echo travis_fold":"end:crashlog \;
  
    # attempt to debug anything killed by the oom killer on linux, just to see if
    # it happened
@@@ -293,7 -286,6 +293,7 @@@ before_deploy
            rm -rf obj/build/dist/doc &&
            cp -r obj/build/dist/* deploy/$TRAVIS_COMMIT;
        fi
 +  - travis_retry gem update --system
  
  deploy:
    - provider: s3
@@@ -322,7 -314,7 +322,7 @@@
        secure: "kUGd3t7JcVWFESgIlzvsM8viZgCA9Encs3creW0xLJaLSeI1iVjlJK4h/2/nO6y224AFrh/GUfsNr4/4AlxPuYb8OU5oC5Lv+Ff2JiRDYtuNpyQSKAQp+bRYytWMtrmhja91h118Mbm90cUfcLPwkdiINgJNTXhPKg5Cqu3VYn0="
      on:
        branch: try
-       condition: $DEPLOY = 1
+       condition: $DEPLOY_ALT = 1
  
    # this is the same as the above deployment provider except that it uploads to
    # a slightly different directory and has a different trigger