[lld/mac] Don't crash on absolute symbols in order files

[thakis]

Summary

Absolute symbols have a nullptr isec. buildInputSectionPriorities()
would defer isec, causing crashes. Ordering absolute symbols doesn't
make sense, so just ignore them. This seems to match ld64.

[int3]

Good catch!

lld/MachO/Writer.cpp

 // contains.
 static DenseMap<const InputSection *, size_t> buildInputSectionPriorities() {
   DenseMap<const InputSection *, size_t> sectionPriorities;
 
   if (config->priorities.empty())
     return sectionPriorities;
 
   auto addSym = [&](Defined &sym) {
+    if (sym.isAbsolute())
+      return;
+
     auto it = config->priorities.find(sym.getName());
     if (it == config->priorities.end())
       return;
 
     SymbolPriorityEntry &entry = it->second;
     size_t &priority = sectionPriorities[sym.isec];
     priority =
         std::max(priority, getSymbolPriority(entry, sym.isec->getFile()));

lld/test/MachO/order-file.s

 # REQUIRES: x86
 # RUN: rm -rf %t; split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/foo.s -o %t/foo.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/abs.s -o %t/abs.o
 # RUN: llvm-ar rcs %t/foo.a %t/foo.o
 
 # FOO-FIRST: <_bar>:
 # FOO-FIRST: <_main>:
 
 # FOO-SECOND: <_main>:
 # FOO-SECOND: <_bar>:
 

 ## symbols appear in an order file, the location in question should be ordered
 ## according to the lowest-ordered symbol that references it.
 
 # RUN: %lld -lSystem -o %t/test-alias %t/test.o %t/foo.o -order_file %t/ord-alias
 # RUN: llvm-objdump -d %t/test-alias | FileCheck %s --check-prefix=FOO-FIRST
 # RUN: %lld -lSystem -o %t/test-alias %t/foo.o %t/test.o -order_file %t/ord-alias
 # RUN: llvm-objdump -d %t/test-alias | FileCheck %s --check-prefix=FOO-FIRST
 
+## Absolute in symbols in order files make no sense. Just ignore them.
+# RUN: %lld -lSystem -dylib -o %t/test-abs %t/abs.o -order_file %t/ord-abs
+
 #--- ord-1
 -[Foo doFoo:andBar:] # just a comment
 _main # another comment
 
 #--- ord-2
 _main # just a comment
 -[Foo doFoo:andBar:] # another comment
 

 _main
 foo.o:-[Foo doFoo:andBar:]
 
 #--- ord-alias
 _bar
 _main
 -[Foo doFoo:andBar:]
 
+#--- ord-abs
+_abs
+
 #--- foo.s
 .globl "-[Foo doFoo:andBar:]"
 "-[Foo doFoo:andBar:]":
 _bar:
   ret
 
 #--- test.s
 .globl _main
 
 _main:
   callq "-[Foo doFoo:andBar:]"
   ret
 
 .section  __DWARF,__debug_aranges,regular,debug
 ltmp1:
   .byte 0
+
+#--- abs.s
+_abs = 42