[clang] [clang] Avoid assertion on invalid member template specialization (PR #201506)

[via cfe-commits]

github-actions[bot] wrote:

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

* 54028 tests passed
* 1317 tests skipped
* 1 test failed

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

### Clang
<details>
<summary>Clang.SemaTemplate/instantiate-member-template.cpp</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\23\include -nostdsysteminc -fsyntax-only -verify C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-member-template.cpp
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\23\include' -nostdsysteminc -fsyntax-only -verify 'C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-member-template.cpp'
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-member-template.cpp Line 266: out-of-line definition of 'B' does not match any declaration in 'A<int>'
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-member-template.cpp Line 266: out-of-line definition of 'B' does not match any declaration in 'GH201490::A<int>'
# | 2 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/201506