[clang] [clang-tools-extra] [clang] NFC: Clear some uses of MemberPointerType::getClass (PR #131965)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

hi, this breaks
`clang -cc1 -triple x86_64-pc-windows-msvc19.26.0 -fsyntax-only  -fms-extensions -fms-compatibility -fms-compatibility-version=19.26 -std=c++17 /tmp/a.ii`

[a.ii.txt](https://github.com/user-attachments/files/19375232/a.ii.txt)

which breaks stage 2 builds of clang on windows, e.g. https://lab.llvm.org/buildbot/#/builders/124/builds/513

https://github.com/llvm/llvm-project/pull/131965