[clang] [FMV] Allow fmv without default declaration. (PR #85454)

[Alexandros Lamprineas via cfe-commits]

labrinea wrote:

Update: The ACLE spec is now explicit about this (see https://github.com/ARM-software/acle/pull/310), so we can safely implement the change in the compiler. Implementation-wise I have moved the static cast to `llvm::Function` inside the `createFunction()` lambda.

https://github.com/llvm/llvm-project/pull/85454