[yaml2obj][obj2yaml] - Change how symbol's binding is descibed when parsing/dumping.

[yaml2obj][obj2yaml] - Change how symbol's binding is descibed when parsing/dumping.

Currently, YAML has the following syntax for describing the symbols:

Symbols:
  Local:
    LocalSymbol1:
    ...
    LocalSymbol2:
    ...
  ...
  Global:
    GlobalSymbol1:
  ...
  Weak:
  ...
  GNUUnique:

I.e. symbols are grouped by their bindings. That is not very convenient,
because:

It does not allow to set a custom binding, what can be useful for producing
broken/special outputs for test cases. Adding a new binding would require to
change a syntax (what we observed when added GNUUnique recently).

It does not allow to change the order of the symbols in .symtab/.dynsym,
i.e. currently all Local symbols are placed first, then Global, Weak and GNUUnique
are following, but we are not able to change the order.

It is not consistent. Binding is just one of the properties of the symbol,
we do not group them by other properties.

It makes the code more complex that it can be. This patch shows it can be simplified
with the change performed.

The patch changes the syntax to just:

Symbols:
  Symbol1:
  ...
  Symbol2:
  ...
...

With that, we are able to work with the binding field just like with any other symbol property.

Differential revision: https://reviews.llvm.org/D60122

llvm-svn: 357595