[VFABI] Read/Write functions for the VFABI attribute.
The attribute is stored at the FunctionIndex attribute set, with the
The get/set methods of the attribute have assertion to verify that:
- Each name in the attribute is a valid VFABI mangled name.
- Each name in the attribute correspond to a function declared in the module.
Differential Revision: https://reviews.llvm.org/D69976