[BOLT] Change symbol handling for secondary function entries

Summary:
Some functions could be called at an address inside their function body.
Typically, these functions are written in assembly as C/C++ does not
have a multi-entry function concept. The addresses inside a function
body that could be referenced from outside are called secondary entry
points.

In BOLT we support processing functions with secondary/multiple entry
points. We used to mark basic blocks representing those entry points
with a special flag. There was only one problem - each basic block has
exactly one MCSymbol associated with it, and for the most efficient
processing we prefer that symbol to be local/temporary. However, in
certain scenarios, e.g. when running in non-relocation mode, we need
the entry symbol to be global/non-temporary.

We could create global symbols for secondary points ahead of time when
the entry point is marked in the symbol table. But not all such entries
are properly marked. This means that potentially we could discover an
entry point only after disassembling the code that references it, and
it could happen after a local label was already created at the same
location together with all its references. Replacing the local symbol
and updating the references turned out to be an error-prone process.

This diff takes a different approach. All basic blocks are created with
permanently local symbols. Whenever there's a need to add a secondary
entry point, we create an extra global symbol or use an existing one
at that location. Containing BinaryFunction maps a local symbol of a
basic block to the global symbol representing a secondary entry point.
This way we can tell if the basic block is a secondary entry point,
and we emit both symbols for all secondary entry points. Since secondary
entry points are quite rare, the overhead of this approach is minimal.

Note that the same location could be referenced via local symbol from
inside a function and via global entry point symbol from outside.
This is true for both primary and secondary entry points.

(cherry picked from FBD21150193)