[libunwind][PAuthLR] Remove PC offset when using FEAT_PAuthLR (llvm#164224)

When originally introduced to libunwind as part of llvm#112171, FEAT_PAuthLR
had its Call Frame Instruction's (CFI's) in a different location to
other Signing Authentication methods. To incorporate this in libunwind,
a 4 byte offset was introduced to work with this. However, this design
was reversed in llvm#121551 so the CFI's are emitted in the same location as
other methods. When making this change, the offset in libunwind was not
removed, so libunwind's PC value would be incorrect.

As the 4 byte offset is no longer needed, that adjustment can be
removed. results->ptrAuthDiversifier will still be set.

Author: Stylie777

libunwind/src/DwarfParser.hpp

@@ -842,12 +842,10 @@ bool CFI_Parser<A>::parseFDEInstructions(A &addressSpace,
             results->savedRegisters[UNW_AARCH64_RA_SIGN_STATE].value ^ 0x3;
         results->setRegisterValue(UNW_AARCH64_RA_SIGN_STATE, value,
                                   initialState);
-        // When calculating the value of the PC, it is assumed that the CFI
-        // instruction is placed before the signing instruction, however it is
-        // placed after. Because of this, we need to take into account the CFI
-        // instruction is one instruction call later than expected, and reduce
-        // the PC value by 4 bytes to compensate.
-        results->ptrAuthDiversifier = fdeInfo.pcStart + codeOffset - 0x4;
+        // When using Feat_PAuthLR, the PC value needs to be captured so that
+        // during unwinding, the correct PC value is used for re-authentication.
+        // It is assumed that the CFI is placed before the signing instruction.
+        results->ptrAuthDiversifier = fdeInfo.pcStart + codeOffset;
         _LIBUNWIND_TRACE_DWARF(
             "DW_CFA_AARCH64_negate_ra_state_with_pc(pc=0x%" PRIx64 ")\n",
             static_cast<uint64_t>(results->ptrAuthDiversifier));