Fix a compile-time warning

lib/CodeGen/CGBuiltin.cpp:3136:12: warning: variable ‘TblPos’ set but not used [-Wunused-but-set-variable]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204599 91177308-0d34-0410-b5e6-96231b3b80d8