[ELF] Remove unused synthetic sections from script commands

[phosek]

Summary

Script commands are processed before unused synthetic sections are
removed. Therefore, if a linker script matches one of these sections
it'll get emitted as an empty output section because the logic for
removing unused synthetic sections ignores script commands which
could have already matched and captured one of these sections. This
patch fixes that by also removing the unused synthetic sections from
the script commands.

ELF/Writer.cpp

for (InputSectionBase *S : llvm::reverse(InputSections)) {
     if (!SS->empty() || !OS)
       continue;
     if ((SS == InX::Got || SS == InX::MipsGot) && ElfSym::GlobalOffsetTable)
       continue;
     OS->Sections.erase(std::find(OS->Sections.begin(), OS->Sections.end(), SS));
     SS->Live = false;
     // If there are no other sections in the output section, remove it from the
     // output.
-    if (OS->Sections.empty())
+    if (OS->Sections.empty()) {
       V.erase(std::find(V.begin(), V.end(), OS));
+      // Also remove script commands matching the output section.
+      auto &Cmds = Script->Opt.Commands;
+      auto I = std::remove_if(Cmds.begin(), Cmds.end(), [&](BaseCommand *Cmd) {
+        if (auto *OSCmd = dyn_cast<OutputSectionCommand>(Cmd))
+          return OSCmd->Sec == OS;
+        return false;
+      });
+      Cmds.erase(I, Cmds.end());
+    }
   }
 }
 
 // Create output section objects and add them to OutputSections.
 template <class ELFT> void Writer<ELFT>::finalizeSections() {
   Out::DebugInfo = findSection(".debug_info");
   Out::PreinitArray = findSection(".preinit_array");
   Out::InitArray = findSection(".init_array");

test/ELF/linkerscript/unused-synthetic.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS { \
+# RUN:    .got  : { *(.got) } \
+# RUN:    .plt  : { *(.plt) } \
+# RUN:    .text : { *(.text) } \
+# RUN:  }" > %t.script
+# RUN: ld.lld -shared -o %t.so --script %t.script %t.o
+
+# RUN: llvm-objdump -section-headers %t.so | FileCheck %s
+# CHECK-NOT:  .got
+# CHECK-NOT:  .plt
+# CHECK:      .text
+# CHECK-NEXT: .dynsym
+
+.global _start
+_start:
+  nop