This introduces a new cgdata format for stable function maps. The raw
data is embedded in the __llvm_merge section during compile time. This
data can be read and merged using the llvm-cgdata tool, into an indexed
cgdata file. Consequently, the tool is now capable of handling either
outlined hash trees, stable function maps, or both, as they are
orthogonal.
Depends on #112662.
This is a patch for
https://discourse.llvm.org/t/rfc-global-function-merging/82608.
