[clang-tools-extra] [clang] [Clang][NFC] Rename CXXMethodDecl::isPure -> is VirtualPure (PR #78463)

[Paul T Robinson via cfe-commits]

pogo59 wrote:

I think "pure virtual" is a more common phrasing than "virtual pure" so maybe `isPureVirtual` instead? Up to Aaron, though.

https://github.com/llvm/llvm-project/pull/78463