Convert a few assert failures into proper errors.

Fixes PR25944.

llvm-svn: 257697
diff --git a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp
index ee1af02..368c95f 100644
--- a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp
+++ b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCObjectWriter.cpp
@@ -24,8 +24,8 @@
 
 protected:
   // Override MCELFObjectTargetWriter.
-  unsigned GetRelocType(const MCValue &Target, const MCFixup &Fixup,
-                        bool IsPCRel) const override;
+  unsigned getRelocType(MCContext &Ctx, const MCValue &Target,
+                        const MCFixup &Fixup, bool IsPCRel) const override;
 };
 } // end anonymous namespace
 
@@ -106,7 +106,8 @@
   llvm_unreachable("Unsupported absolute address");
 }
 
-unsigned SystemZObjectWriter::GetRelocType(const MCValue &Target,
+unsigned SystemZObjectWriter::getRelocType(MCContext &Ctx,
+                                           const MCValue &Target,
                                            const MCFixup &Fixup,
                                            bool IsPCRel) const {
   MCSymbolRefExpr::VariantKind Modifier = Target.getAccessVariant();