[BOLT] Basic support for split functions

Summary:
This adds very basic and limited support for split functions.
In non-relocation mode, split functions are ignored, while their debug
info is properly updated. No support in the relocation mode yet.

Split functions consist of a main body and one or more fragments.
For fragments, the main part is called their parent. Any fragment
could only be entered via its parent or another fragment.

The short-term goal is to correctly update debug information for split
functions, while the long-term goal is to have a complete support
including full optimization. Note that if we don't detect split
bodies, we would have to add multiple entry points via tail calls,
which we would rather avoid.

Parent functions and fragments are represented by a `BinaryFunction`
and are marked accordingly. For now they are marked as non-simple, and
thus only supported in non-relocation mode. Once we start building a
CFG, it should be a common graph (i.e. the one that includes all
fragments) in the parent function.

The function discovery is unchanged, except for the detection of
`\.cold\.` pattern in the function name, which automatically marks the
function as a fragment of another function.

Because of the local function name ambiguity, we cannot rely on the
function name to establish child fragment and parent relationship.
Instead we rely on disassembly processing.

`BinaryContext::getBinaryFunctionContainingAddress()` now returns a
parent function if an address from its fragment is passed.

There's no jump table support at the moment. Jump tables can have
source and destinations in both fragment and parent.

Parent functions that enter their fragments via C++ exception handling
mechanism are not yet supported.

(cherry picked from FBD14970569)