[DWARF] Refactor DWARF classes to use unified error reporting. NFC.

DWARF-related classes in lib/DebugInfo/DWARF contained 
duplicating code for creating StringError instances, like:

template <typename... Ts>
static Error createError(char const *Fmt, const Ts &... Vals) {
  std::string Buffer;
  raw_string_ostream Stream(Buffer);
  Stream << format(Fmt, Vals...);
  return make_error<StringError>(Stream.str(), inconvertibleErrorCode());
}

Similar function was placed in Support lib in https://reviews.llvm.org/D49824

This revision makes DWARF classes use this function
instead of their local implementation of it.

Reviewers: aprantl, dblaikie, probinson, wolfgangp, JDevlieghere, jhenderson

Reviewed By: JDevlieghere, jhenderson

Differential Revision: https://reviews.llvm.org/D49964

llvm-svn: 340163
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp b/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp
index f00141c..43b67a2 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp
@@ -1598,7 +1598,8 @@
   const Target *TheTarget =
       TargetRegistry::lookupTarget(TT.str(), TargetLookupError);
   if (!TargetLookupError.empty())
-    return make_error<StringError>(TargetLookupError, inconvertibleErrorCode());
+    return createStringError(errc::invalid_argument,
+                             TargetLookupError.c_str());
   RegInfo.reset(TheTarget->createMCRegInfo(TT.str()));
   return Error::success();
 }