[lld-macho] Refactor BPSectionOrderer with CRTP. NFC PR #117514 refactored BPSectionOrderer to be used by the ELF port but introduced some inefficiency: * BPSectionBase/BPSymbol are wrappers around a single pointer. The numbers of sections and symbols could be huge, and the extra allocations are memory inefficient. * Reconstructing the returned DenseMap (since BPSectionBase != InputSectin) is wasteful. This patch refactors BPSectionOrderer with Curiously Recurring Template Pattern and eliminates the inefficiency. In addition, `symbolToSectionIdxs` is removed and `rootSymbolToSectionIdxs` building is moved to lld/MachO: while getting sections for symbols is cheap in Mach-O, it is awkward and inefficient in the ELF port. While here, add a file-level comment and replace some `StringMap<*>` (which copies strings) with `DenseMap<CachedHashStringRef, *>`. Pull Request: https://github.com/llvm/llvm-project/pull/124482
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