Function parseAndCorrectEhFrameFDEs

Parse FDEs in an eh_frame section, building a function-address-to-FDE-offset map for compact unwind synthesis. If correctPcRelValues is true (x86-64), also corrects pcrel pc_begin and LSDA fields that LLVM resolved at assembly time; our layout may have changed relative distances. On AArch64, LLVM emits explicit relocations for these, so no correction is needed.