Only build lldb-tblgen if it's not a current target

Summary:
When doing standalone builds, you could potentially be building against
an llvm which also built lldb. If this were the case, you'd be
attempting to build this target twice.

Reviewers: xiaobai

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D64847

llvm-svn: 366394