[gSYM] Add support merged functions in gSYM format (#101604) This patch introduces support for storing debug info for merged functions in the GSYM debug info. It allows GSYM to represent multiple functions that share the same address range, which occur when multiple functions are merged during linker ICF. The core of this functionality is the new `MergedFunctionsInfo` class, which is integrated into the existing `FunctionInfo` structure. During GSYM creation, functions with identical address ranges are now grouped together, with one function serving as the "master" and the others becoming "merged" functions. This organization is preserved in the GSYM format and can be read back and displayed when dumping GSYM information. Old readers will only see the master function, and ther "merged" functions will not be processed. Note: This patch just adds the functionality to the gSYM format - additional changes to the gsym format and algorithmic changes to logic existing tooling are needed to take advantage of this data. Exact output of `llvm-gsymutil --verify --verbose` for the included test: [gist](https://gist.github.com/alx32/b9c104d7f87c0b3e7b4171399fc2dca3)
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