[clang] [clang][Sema] Crash due to asm labeled incomplete global register decls (PR #219746)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 56327 tests passed
* 1553 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### Clang
<details>
<summary>Clang.Sema/global-explicit-register-undefined-type.c</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
c:\_work\llvm-project\llvm-project\build\bin\clang.exe -cc1 -internal-isystem C:\_work\llvm-project\llvm-project\build\lib\clang\24\include -nostdsysteminc C:\_work\llvm-project\llvm-project\clang\test\Sema\global-explicit-register-undefined-type.c -triple arm64-apple-macosx -fsyntax-only -verify
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\24\include' -nostdsysteminc 'C:\_work\llvm-project\llvm-project\clang\test\Sema\global-explicit-register-undefined-type.c' -triple arm64-apple-macosx -fsyntax-only -verify
# .---command stderr------------
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\Sema\global-explicit-register-undefined-type.c Line 11: register 'x1' unsuitable for global register variables on this target
# |   File C:\_work\llvm-project\llvm-project\clang\test\Sema\global-explicit-register-undefined-type.c Line 17: register 'x1' unsuitable for global register variables on this target
# |   File C:\_work\llvm-project\llvm-project\clang\test\Sema\global-explicit-register-undefined-type.c Line 17: unsupported type for named register variable
# | 3 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/219746