[NFC][Clang][AArch64]Refactor implementation of Neon vectors MFloat8… (#114804)

…x8 and MFloat8x16

This patch adds MFloat8 as a TypeFlag and Kind on Neon to generate the
typedefs using emitNeonTypeDefs.
It does not need any change in Clang, because SEMA and CodeGen use the
Builtins defined in AArch64SVEACLETypes.def