[lldb][TypeSystemClang] Format pointers to member functions as eFormatHex

Before this patch, LLDB used to format pointers to members, such as,
```
void (Foo::*pointer_to_member_func)() = &Foo::member_func;
```
as `eFormatBytes`. E.g.,
```
(lldb) v pointer_to_member_func
(void (Foo::*)()) $1 = 94 3f 00 00 01 00 00 00 00 00 00 00 00 00 00 00
```

This patch makes sure we format pointers to member functions the same
way we do regular function pointers.

After this patch we format member pointers as:
```
(lldb) v pointer_to_member_func
(void (Foo::*)()) ::pointer_to_member_func = 0x00000000000000000000000100003f94
```

Differential Revision: https://reviews.llvm.org/D145241