[llvm/Object] - Convert SectionRef::getName() to return Expected<>

SectionRef::getName() returns std::error_code now.
Returning Expected<> instead has multiple benefits.

For example, it forces user to check the error returned.
Also Expected<> may keep a valuable string error message,
what is more useful than having a error code.
(Object\invalid.test was updated to show the new messages printed.)

This patch makes a change for all users to switch to Expected<> version.

Note: in a few places the error returned was ignored before my changes.
In such places I left them ignored. My intention was to convert the interface
used, and not to improve and/or the existent users in this patch.
(Though I think this is good idea for a follow-ups to revisit such places
and either remove consumeError calls or comment each of them to clarify why
it is OK to have them).

Differential revision: https://reviews.llvm.org/D66089

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368812 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Object/ELFObjectFile.cpp

@@ -392,9 +392,13 @@ ELFObjectFileBase::getPltAddresses() const {
     return {};
   Optional<SectionRef> Plt = None, RelaPlt = None, GotPlt = None;
   for (const SectionRef &Section : sections()) {
-    StringRef Name;
-    if (Section.getName(Name))
+    Expected<StringRef> NameOrErr = Section.getName();
+    if (!NameOrErr) {
+      consumeError(NameOrErr.takeError());
       continue;
+    }
+    StringRef Name = *NameOrErr;
+
     if (Name == ".plt")
       Plt = Section;
     else if (Name == ".rela.plt" || Name == ".rel.plt")