[clang] [clang][Builtins] Parse clang extended vectors types. (PR #83584)

[Francesco Petrogalli via cfe-commits]

fpetrogalli wrote:

@francisvm / @philnik777 - I have opted for:

    '_ExtVector<' <lanes> ',' <scalar type> '>'

The reason for specifying <lanes> first is due to the fact that it makes it simpler to build the final string, because the `ParseType` function is recursive.

https://github.com/llvm/llvm-project/pull/83584