[clang] [CLANG]Add Scalable vectors for mfloat8_t (PR #101644)

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CarolineConcatto wrote:

Hi @paulwalker-arm,
I believe I addressed all the comments in the code. 

I updated almost all of them to have a test for the SVMfloat8_t.
The ones that are not updated are the ones that test FIXED SVE types. 
The mangling needs scalar type support, BuiltinType::MFloat8,  in order to NOT mangle to SVUint8 (CXXNameMangler::mangleAArch64FixedSveVectorType(const VectorType *T))
I believe I can add support for that when the scalar type is merged.
About the tuple types.
If you dont mind I would rather address that in another patch.

https://github.com/llvm/llvm-project/pull/101644