[ELF] Add a corrector for case mismatch problems

[MaskRay]

Internally we have a user who sees undefined reference to ...::UpdateToGet when they define UpdateToGET in a .cc file while declaring the function UpdateToGet in a .h file. It was suggested that such case mismatch spell corrector can be helpful.

[MaskRay]

Fix a comment...

[peter.smith]

I think this is a good idea, I'd say most of my typos are of this form, such as SubTarget vs Subtarget. Will be worth waiting to see if anyone in US timezone has any objections.

[grimar]

LGTM too.

[MaskRay]

Rebase after we decide to use symtab->symbols()

[thakis]

This one looks like it would make sense in the COFF linker too.

lld/ELF/Relocations.cpp

for (size_t i = 0, e = name.size(); i != e + 1; ++i) {
     }
 
     // Delete name[i].
     newName = (name.substr(0, i) + name.substr(i + 1)).str();
     if (const Symbol *s = suggest(newName))
       return s;
   }
 
+  // Case mismatch, e.g. Foo vs FOO.
+  for (auto &it : map)
+    if (name.equals_lower(it.first))
+      return it.second;
+  for (Symbol *sym : *symtab)
+    if (!sym->isUndefined() && name.equals_lower(sym->getName()))
+      return sym;
+
   // The reference may be a mangled name while the definition is not. Suggest a
   // missing extern "C".
   if (name.startswith("_Z")) {
     std::string buf = name.str();
     llvm::ItaniumPartialDemangler d;
     if (!d.partialDemangle(buf.c_str()))
       if (char *buf = d.getFunctionName(nullptr, nullptr)) {
         const Symbol *s = suggest(buf);

lld/test/ELF/undef-spell-corrector.s

 ## Local defined symbols.
 # RUN: echo '_Z3fooPKi: call _Z3fooPi' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
 # RUN: not ld.lld %t1.o -o /dev/null 2>&1 | FileCheck --check-prefix=CONST %s
 
 # CONST:      error: undefined symbol: foo(int*)
 # CONST-NEXT: >>> referenced by {{.*}}
 # CONST-NEXT: >>> did you mean: foo(int const*)
 
+## Case mismatch.
+# RUN: echo 'call _Z3FOOPKi' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
+# RUN: not ld.lld %t.o %t1.o -o /dev/null 2>&1 | FileCheck --check-prefix=CASE %s
+# RUN: echo '_Z3fooPKi: call _Z3FOOPKi' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
+# RUN: not ld.lld %t1.o -o /dev/null 2>&1 | FileCheck --check-prefix=CASE %s
+
+# CASE:      error: undefined symbol: FOO(int const*)
+# CASE-NEXT: >>> referenced by {{.*}}
+# CASE-NEXT: >>> did you mean: foo(int const*)
+
 .globl _start, abcde, _Z3fooPKi
 _start:
 abcde:
 _Z3fooPKi: