[clang] [Clang][AArch64] Use __clang_arm_builtin_alias for overloaded svreinterpret's (PR #92427)

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rsandifo-arm wrote:

> > Thinking about it a bit more, maybe we can just do some magic to make things work? Say, if you specify `__attribute__((target("sve"))) __arm_streaming_compatible`, and the caller is in streaming mode, allow the call even if the caller doesn't have SVE proper.
> 
> Thinking a bit more, this is probably not quite what we want: even if the function body itself is streaming compatible, it might call non-streaming functions that require SVE. Maybe spell this something like `__attribute__((target("sve-or-streaming"))) __arm_streaming_compatible`.

I suppose the idea here is that:
```
   __attribute__((target("sve"))) void f() { … }
   void g() { … f(); … }
```
should be diagnosed, on the basis that, when compiled with default flags, `g` doesn't guarantee the availability of SVE, whereas `f` requires it?  If so, I don't think we should do that, for two reasons:

First, it's IMO valid to do:
```
   __attribute__((target("sve"))) void sve_routine() { … }
   void main_interface() {
     if (SVE_is_available() && problem_has_certain_characteristics())
       sve_routine();
     else
       …
   }
```
That is, feature gating can be dynamic.  It doesn't need to be a load-time thing.

Second, at least in GCC, the `target` attribute is not part of a function's type, so it's not an error to do:
```
foo.h: void f();
foo.cc: __attribute__((target("sve"))) void f() { … }
```
And I'd argue that that's a feature rather than a bug.  It allows load-time selection of DSOs based on the target.

https://github.com/llvm/llvm-project/pull/92427