[MemProf] Context disambiguation cloning pass [patch 1a/3]

[MemProf] Context disambiguation cloning pass [patch 1a/3]

Support for building, printing, and displaying CallsiteContextGraph
which represents the MemProf metadata contexts. Uses CRTP to enable
support for both IR (regular LTO) and summary (ThinLTO). This patch
includes the support for building it in regular LTO mode (from
memprof and callsite metadata), and the next patch will add the
handling for building it from ThinLTO summaries.

Also includes support for dumping the graph to text and to dot files.

Follow-on patches will contain the support for cloning on the graph and
in the IR.

The graph represents the call contexts in all memprof metadata on
allocation calls, with nodes for the allocations themselves, as well as
for the calls in each context. The graph is initially built from the
allocation memprof metadata (or summary) MIBs. It is then updated to
match calls with callsite metadata onto the nodes, updating it to
reflect any inlining performed on those calls.

Each MIB (representing an allocation's call context with allocation
behavior) is assigned a unique context id during the graph build. The
edges and nodes in the graph are decorated with the context ids they
carry. This is used to correctly update the graph when cloning is
performed so that we can uniquify the context for a single (possibly
cloned) allocation.

Depends on D140786.

Differential Revision: https://reviews.llvm.org/D140908