[ELF] Fix compareSections assertion failure when OutputDescs in sectionCommands are non-contiguous

In a `--defsym y0=0 -T a.lds` link where a.lds contains only INSERT
commands, the `script->sectionCommands` layout may be:
```
orphan sections
SymbolAssignment due to --defsym
sections created by INSERT commands
```

The `OutputDesc` objects are not contiguous in sortInputSections, and
`compareSections` will be called with a SymbolAssignment argument,
leading to an assertion failure.

(cherry picked from commit dee8786)

Author: MaskRay

lld/ELF/Writer.cpp

@@ -1518,12 +1518,12 @@ template <class ELFT> void Writer<ELFT>::sortSections() {
     if (auto *osd = dyn_cast<OutputDesc>(cmd))
       osd->osec.sortRank = getSectionRank(osd->osec);
   if (!script->hasSectionsCommand) {
-    // We know that all the OutputSections are contiguous in this case.
-    auto isSection = [](SectionCommand *cmd) { return isa<OutputDesc>(cmd); };
-    std::stable_sort(
-        llvm::find_if(script->sectionCommands, isSection),
-        llvm::find_if(llvm::reverse(script->sectionCommands), isSection).base(),
-        compareSections);
+    // OutputDescs are mostly contiguous, but may be interleaved with
+    // SymbolAssignments in the presence of INSERT commands.
+    auto mid = std::stable_partition(
+        script->sectionCommands.begin(), script->sectionCommands.end(),
+        [](SectionCommand *cmd) { return isa<OutputDesc>(cmd); });
+    std::stable_sort(script->sectionCommands.begin(), mid, compareSections);
   }
 
   // Process INSERT commands and update output section attributes. From this

lld/test/ELF/linkerscript/insert-before.test

@@ -24,8 +24,9 @@
 ## without making more layout changes. Address/offset assignments are different
 ## with a main linker script.
 
-# RUN: ld.lld --script %s %t1.o -o %t2
-# RUN: llvm-readelf -S -l %t2 | FileCheck --check-prefix=CHECK2 %s
+## Test non-contiguous OutputDescs in script->sectionCommands.
+# RUN: ld.lld --defsym y0=1 %s --defsym y1=1 %t1.o -o %t2
+# RUN: llvm-readelf -S -l -sX %t2 | FileCheck --check-prefix=CHECK2 %s
 # CHECK2:      Name      Type     Address            Off      Size   ES Flg
 # CHECK2-NEXT:           NULL
 # CHECK2-NEXT: .foo.text PROGBITS 000000000020{{.*}} [[#%x,]] 000008 00  AX
@@ -40,9 +41,13 @@
 # CHECK2-NEXT: LOAD      {{.*}} RW  0x1000
 # CHECK2-NEXT: GNU_STACK {{.*}} RW  0
 
+# CHECK2:      NOTYPE  GLOBAL DEFAULT  ABS               y0
+# CHECK2:      NOTYPE  GLOBAL DEFAULT  [[#]] (.foo.text) x0
+# CHECK2:      NOTYPE  GLOBAL DEFAULT  ABS               y1
+
 SECTIONS { .byte : { BYTE(0) } } INSERT BEFORE .data;
 
 SECTIONS { .foo.data : { *(.foo.data) } } INSERT BEFORE .data;
 
 ## The input section .foo.text is an orphan. It will be placed in .foo.text
-SECTIONS { .foo.text : {} } INSERT BEFORE .text;
+SECTIONS { .foo.text : { x0 = .; } } INSERT BEFORE .text;