[ELF] Filter out non InputSection members from InputSections

[phosek]

Summary

InputSections may contain MergeInputSection members which trigger a segmentation fault when trying to cast them to InputSection.

[ruiu]

LGTM

[ruiu]

Can you add a test

[phosek]

Added a test case.

ELF/LinkerScript.cpp

 LinkerScript::computeInputSections(const InputSectionDescription *Cmd) {
   std::vector<InputSection *> Ret;
 
   // Collects all sections that satisfy constraints of Cmd.
   for (const SectionPattern &Pat : Cmd->SectionPatterns) {
     size_t SizeBefore = Ret.size();
 
     for (InputSectionBase *Sec : InputSections) {
+      if (!isa<InputSection>(Sec))
+        continue;
+
       if (Sec->Assigned)
         continue;
 
       // For -emit-relocs we have to ignore entries like
       //   .rela.dyn : { *(.rela.data) }
       // which are common because they are in the default bfd script.
       if (Sec->Type == SHT_REL || Sec->Type == SHT_RELA)
         continue;

test/ELF/linkerscript/merge-sections.s

 
 # CHECK:      Name: begin
 # CHECK-NEXT: Value: 0x[[ADDR1]]
 
 # CHECK:      Name: end
 # 0x19E = begin + sizeof(.foo) = 0x190 + 0xE
 # CHECK-NEXT: Value: 0x19E
 
+# Check that we don't crash with --gc-sections
+# RUN: ld.lld -gc-sections -o %t2 --script %t.script %t -shared
+# RUN: llvm-readobj -s -t %t2 | FileCheck %s --check-prefix=GC
+
+# GC:        Name: .foo
+# GC-NEXT:   Type: SHT_PROGBITS
+# GC-NEXT:   Flags [
+# GC-NEXT:     SHF_ALLOC
+# GC-NEXT:   ]
+
 .section        .foo.1a,"aMS",@progbits,1
 .asciz "foo"
 
 .section        .foo.1b,"aMS",@progbits,1
 .asciz "foo"
 
 .section        .foo.2a,"aM",@progbits,1
 .byte 42