[SHT_LLVM_BB_ADDR_MAP] Encode and decode callsite offsets in a newly-introduced SHT_LLVM_BB_ADDR_MAP version. (#144426) Recently, we have been looking at some optimizations targeting individual calls. In particular, we plan to extend the address mapping technique to map to individual callsites. For example, in this piece of code for a basic blocks: ``` <BB>: 1200: lea 0x1(%rcx), %rdx 1204: callq foo 1209: cmpq 0x10, %rdx 120d: ja L1 ``` We want to emit 0x9 as the call site offset for `callq foo` (the offset from the block entry to right after the call), so that we know if a sampled address is before the call or after. This PR implements the decode/encode/emit capability. The Codegen change will be implemented in a later PR.
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