Support underscore as constant name

[dsciarra]

Issue: #54912

[rust-highfive]

r? @pnkfelix

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[rust-highfive]

The job x86_64-gnu-llvm-5.0 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.

[00:49:28] .............................................................................i...................... 4200/4593
[00:49:31] .................................................................................................... 4300/4593
[00:49:35] ..................................................................................................F. 4400/4593
[00:49:37] .................................................................................................... 4500/4593
 "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/underscore_const_names/auxiliary" "-A" "unused"
[00:49:40] ------------------------------------------
[00:49:40] 
[00:49:40] ------------------------------------------
[00:49:40] stderr:
---
[00:49:40] 
[00:49:40] thread 'main' panicked at 'Some tests failed', tools/compiletest/src/main.rs:501:22
[00:49:40] 
[00:49:40] 
[00:49:40] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "-

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:00dcbd40
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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)

[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit 406cbf1 has been approved by petrochenkov

[bors]

⌛ Testing commit 406cbf1 with merge 2462a2d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: petrochenkov
Pushing 2462a2d to master...