[FMV] Allow multi versioning without default declaration. (#85454)

This was a limitation which has now been lifted. Please read the
thread below for more details:

https://github.com/llvm/llvm-project/pull/84405#discussion_r1525583647

Basically it allows to separate versioned implementations across
different TUs without having to share private header files which
contain the default declaration.

The ACLE spec has been updated accordingly to make this explicit:
"Each version declaration should be visible at the translation
 unit in which the corresponding function version resides."

https://github.com/ARM-software/acle/pull/310

If a resolver is required (because there is a caller in the TU),
then a default declaration is implicitly generated.