Make MCAsmParserSemaCallback::LookupInlineAsmLabel a pure virtual function

Summary:
r218229 made this function return a dummy nullptr in order to avoid
API breakage between clang/llvm.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5432

llvm-svn: 218266

include/llvm/MC/MCParser/MCAsmParser.h

@@ -55,8 +55,7 @@ public:
                                           InlineAsmIdentifierInfo &Info,
                                           bool IsUnevaluatedContext) = 0;
   virtual StringRef LookupInlineAsmLabel(StringRef Identifier, SourceMgr &SM,
-                                         SMLoc Location, bool Create)
-  { return nullptr; }
+                                         SMLoc Location, bool Create) = 0;
 
   virtual bool LookupInlineAsmField(StringRef Base, StringRef Member,
                                     unsigned &Offset) = 0;