[clang] Allow 'nomerge' attribute for function pointers

Allow specifying 'nomerge' attribute for function pointers,
e.g. like in the following C code:

    extern void (*foo)(void) __attribute__((nomerge));
    void bar(long i) {
      if (i)
        foo();
      else
        foo();
    }

With the goal to attach 'nomerge' to both calls done through 'foo':

    @foo = external local_unnamed_addr global ptr, align 8
    define dso_local void @bar(i64 noundef %i) local_unnamed_addr #0 {
      ; ...
      %0 = load ptr, ptr @foo, align 8, !tbaa !5
      ; ...
    if.then:
      tail call void %0() #1
      br label %if.end
    if.else:
      tail call void %0() #1
      br label %if.end
    if.end:
      ret void
    }
    ; ...
    attributes #1 = { nomerge ... }

Report a warning in case if 'nomerge' is specified for a variable that
is not a function pointer, e.g.:

    t.c:2:22: warning: 'nomerge' attribute is ignored because 'j' is not a function pointer [-Wignored-attributes]
        2 | int j __attribute__((nomerge));
          |                      ^

The intended use-case is for BPF backend.

BPF provides a sort of "standard library" functions that are called
helpers. BPF also verifies usage of these helpers before program
execution. Because of limitations of verification / runtime model it
is important to keep calls to some of such helpers from merging.

An example could be found by the link [1], there input C code:

     if (data_end - data > 1024) {
         bpf_for_each_map_elem(&map1, cb, &cb_data, 0);
     } else {
         bpf_for_each_map_elem(&map2, cb, &cb_data, 0);
     }

Is converted to bytecode equivalent to:

     if (data_end - data > 1024)
       tmp = &map1;
     else
       tmp = &map2;
     bpf_for_each_map_elem(tmp, cb, &cb_data, 0);

However, BPF verification/runtime requires to use the same map address
for each particular `bpf_for_each_map_elem()` call.

The 'nomerge' attribute is a perfect match for this situation, but
unfortunately BPF helpers are declared as pointers to functions:

    static long (*bpf_for_each_map_elem)(void *map, ...) = (void *) 164;

Hence, this commit, allowing to use 'nomerge' for function pointers.

[1] https://lore.kernel.org/bpf/03bdf90f-f374-1e67-69d6-76dd9c8318a4@meta.com/

Differential Revision: https://reviews.llvm.org/D152986