[clang] [ASTMatchers] fix captureVars assertion failure on capturesVariables (PR #76619)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you for the fix, can you add more details to your summary. The summary is what usually goes into the git log. We would like those to be as descriptive as possible to avoid having to do extra digging to understand the change at a high level. 

https://github.com/llvm/llvm-project/pull/76619