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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I might suggest `isPureVirtual` as it better reflects the 'term of art' (https://en.cppreference.com/w/cpp/language/abstract_class see 'A pure virtual function is a....).
> 
> Else LG and SG TM.

I have the same preference for `isPureVirtual` but otherwise LGTM.

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