[clang] [clang] pop explicit to keep context stack balance (PR #77312)

[Qizhi Hu via cfe-commits]

jcsxky wrote:

> Can you explain why this change is needed, and provide a test case? (in `clang/test/Parser`) Thanks

Test case has been added and update description.

https://github.com/llvm/llvm-project/pull/77312