[GSYM] Callsites: Add data format support and loading from YAML (#109781)

This PR adds support in the gSYM format for call site information and
adds support for loading call sites from a YAML file. The support for
YAML input is mostly for testing purposes - so we have a way to test the
functionality.

Note that this data is not currently used in the gSYM tooling - the
logic to use call sites will be added in a later PR.

The reason why we need call site information in gSYM files is so that we
can support better call stack function disambiguation in the case where
multiple functions have been merged due to optimization (linker ICF).
When resolving a merged function on the callstack, we can use the call
site information of the calling function to narrow down the actual
function that is being called, from the set of all merged functions.

See [this
RFC](https://discourse.llvm.org/t/rfc-extending-gsym-format-with-call-site-information-for-merged-function-disambiguation/80682)
for more details on this change.

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