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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218266 91177308-0d34-0410-b5e6-96231b3b80d8