[ELF] - Do not crash if user section has name equal to one of synthetic sections.

[grimar]

Summary

Previously we just crashed when had user defined
section .shstrtab, for example. Which name equals to synthetic one,
but have different type.

Testcase reveals an issue.

[ruiu]

ELF/InputSection.cpp

38–41

If a section is a synthetic section, it's output will be something like

'foo'

while if it is not, it will be something like

bar.o:(foo)

So, apparently they are inconsistent in quoting style. That's not good.

ELF/InputSection.cpp

 using namespace llvm::support::endian;
 
 using namespace lld;
 using namespace lld::elf;
 
 // Returns a string to construct an error message.
 template <class ELFT>
 std::string lld::toString(const InputSectionBase<ELFT> *Sec) {
+  // File can absent if section is synthetic.
+  if (!Sec->getFile())
+    return ("'" + Sec->Name + "'").str();
   return (Sec->getFile()->getName() + ":(" + Sec->Name + ")").str();
 }
 
 template <class ELFT>
 static ArrayRef<uint8_t> getSectionContents(elf::ObjectFile<ELFT> *File,
                                             const typename ELFT::Shdr *Hdr) {
   if (!File || Hdr->sh_type == SHT_NOBITS)
     return makeArrayRef<uint8_t>(nullptr, Hdr->sh_size);
   return check(File->getObj().getSectionContents(Hdr));

test/ELF/incompatible-section-types2.s

+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: not ld.lld %t.o -o %t 2>&1 | FileCheck %s
+
+// CHECK: error: Section has different type from others with the same name '.shstrtab'
+
+.section .shstrtab,""
+.short 20