Adds the ability to lookup and display all merged functions for an
address in llvm-gsymutil.
Now, when `--merged-functions` is used in combination with
`--address/--addresses-from-stdin`, lookup results will contain
information about merged functions, if available.
To support printing merged function information when using the
`--verbose` option, the `LookupResult` data structure also had to be
extended with pointers to the raw function data and raw merged function
data. This is because merged functions share the same address range, so
it's not easy to look up the raw merged function data for a particular
`LookupResult` that is based on a merged function.
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