[Sema][SVE] Allow casting SVE types to themselves in C

Casts from an SVE type to itself aren't very useful, but they are
supposed to be valid, and could occur in things like macro expansions.

Such casts already work for C++ and are tested by sizeless-1.cpp.
This patch makes them work for C too.

Differential Revision: https://reviews.llvm.org/D76694