[llvm][clang][IFS] Enhancing the llvm-ifs yaml format for symbol lists.

Prior to this change the clang interface stubs format resembled
something ending with a symbol list like this:

 Symbols:
   a: { Type: Func }

This was problematic because we didn't actually want a map format and
also because we didn't like that an empty symbol list required
"Symbols: {}". That is to say without the empty {} llvm-ifs would crash
on an empty list.

With this new format it is much more clear which field is the symbol
name, and instead the [] that is used to express an empty symbol vector
is optional, ie:

Symbols:
 - { Name: a, Type: Func }

or

Symbols: []

or

Symbols:

This further diverges the format from existing llvm-elftapi. This is a
good thing because although the format originally came from the same
place, they are not the same in any way.

Differential Revision: https://reviews.llvm.org/D76979