Make single-variant "c-like" enums have no size.

[Aatch]

This makes an enum like enum Foo { Foo } be zero-sized. If a
discriminant is set to non-zero, it still treats it as a c-like enum.

Fixes #15747

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[luqmana]

r=me with test.

[Aatch]

@luqmana added, despite knowing it will conflict another one of my pull requests >.>

[alexcrichton]

I think this has a failure in the debuginfo tests:

failures:

---- [debuginfo-gdb] debuginfo/c-style-enum.rs stdout ----
        NOTE: compiletest thinks it is using GDB version 7.7

error: line not found in debugger output: $1 = TheOnlyVariant

status: exit code: 0
command: gdb -quiet -batch -nx -command=x86_64-unknown-linux-gnu/test/debuginfo-gdb/c-style-enum.debugger.script
stdout:
------------------------------------------
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".
Breakpoint 1 at 0x988: file src/test/debuginfo/c-style-enum.rs, line 156.

------------------------------------------
stderr:
------------------------------------------
x86_64-unknown-linux-gnu/test/debuginfo-gdb/c-style-enum.debugger.script:8: Error in sourced command file:
No symbol "c-style-enum::SINGLE_VARIANT" in current context.

------------------------------------------

thread '[debuginfo-gdb] debuginfo/c-style-enum.rs' panicked at 'explicit panic', /home/alex/code/rust3/src/compiletest/runtest.rs:1487



failures:
    [debuginfo-gdb] debuginfo/c-style-enum.rs

test result: FAILED. 91 passed; 1 failed; 4 ignored; 0 measured

[alexcrichton]

Closing due to inactivity, but feel free to reopen with a rebase!