[lld/mac] Don't assert on -dead_strip + arm64 range extension thunks

[thakis]

Summary

The assert is harmless and thinks worked fine in builds with asserts enabled,
but it's still nice to fix the assert.

[int3]

lgtm

[thakis]

Thanks!

lld/MachO/ConcatOutputSection.cpp

 // * During address assignment, MergedInputSection::finalize() examines call
 //   sites by ascending address and creates thunks.  When a function is beyond
 //   the range of a call site, we need a thunk. Place it at the largest
 //   available forward address from the call site. Call sites increase
 //   monotonically and thunks are always placed as far forward as possible;
 //   thus, we place thunks at monotonically increasing addresses. Once a thunk
 //   is placed, it and all previous input-section addresses are final.
 //
-// * MergedInputSection::finalize() and MergedInputSection::writeTo() merge
+// * ConcatInputSection::finalize() and ConcatInputSection::writeTo() merge
 //   the inputs and thunks vectors (both ordered by ascending address), which
 //   is simple and cheap.
 
 DenseMap<Symbol *, ThunkInfo> lld::macho::thunkMap;
 
 // Determine whether we need thunks, which depends on the target arch -- RISC
 // (i.e., ARM) generally does because it has limited-range branch/call
 // instructions, whereas CISC (i.e., x86) generally doesn't. RISC only needs
for (Reloc &r : reverse(relocs)) {
         // distance between callVA and highVA, then shift some thunks
         // to occupy address-space formerly occupied by the
         // unfinalized inputs[finalIdx].
         fatal(Twine(__FUNCTION__) + ": FIXME: thunk range overrun");
       }
       thunkInfo.isec =
           make<ConcatInputSection>(isec->getSegName(), isec->getName());
       thunkInfo.isec->parent = this;
+
+      // This code runs after dead code removal. Need to set the `live` bit
+      // on the thunk isec so that asserts that check that only live sections
+      // get written are happy.
+      thunkInfo.isec->live = true;
+
       StringRef thunkName = saver.save(funcSym->getName() + ".thunk." +
                                        std::to_string(thunkInfo.sequence++));
       r.referent = thunkInfo.sym = symtab->addDefined(
           thunkName, /*file=*/nullptr, thunkInfo.isec, /*value=*/0,
           /*size=*/thunkSize, /*isWeakDef=*/false, /*isPrivateExtern=*/true,
           /*isThumb=*/false, /*isReferencedDynamically=*/false,
           /*noDeadStrip=*/false);
       target->populateThunk(thunkInfo.isec, funcSym);

lld/test/MachO/arm64-thunks.s

 # REQUIRES: aarch64
 
 ## Check for the following:
 ## (1) address match between thunk definitions and call destinations
-## (2) address match between thunk page+offset computations and function definitions
+## (2) address match between thunk page+offset computations and function
+##     definitions
 ## (3) a second thunk is created when the first one goes out of range
-## (4) early calls to a dylib stub use a thunk, and later calls the stub directly
+## (4) early calls to a dylib stub use a thunk, and later calls the stub
+##     directly
 ## Notes:
 ## 0x4000000 = 64 Mi = half the magnitude of the forward-branch range
 
 # RUN: rm -rf %t; mkdir %t
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t/input.o
-# RUN: %lld -arch arm64 -lSystem -o %t/thunk %t/input.o
+# RUN: %lld -arch arm64 -dead_strip -lSystem -o %t/thunk %t/input.o
 # RUN: llvm-objdump -d --no-show-raw-insn %t/thunk | FileCheck %s
 
 # CHECK: Disassembly of section __TEXT,__text:
 
 # CHECK: [[#%.13x, A_PAGE:]][[#%.3x, A_OFFSET:]] <_a>:
 # CHECK:  bl 0x[[#%x, A:]] <_a>
 # CHECK:  bl 0x[[#%x, B:]] <_b>
 # CHECK:  bl 0x[[#%x, C:]] <_c>