TableGen.cmake: Try to fix build breakage introduce in r305142.

LLVM_TABLEGEN_TARGET is undefined in clang standalone build.
STREQUAL cannot omit LHS. Then I saw an error;

  CMake Error at /path/to/install/llvm/lib/cmake/llvm/TableGen.cmake:40 (if):
      if given arguments:
        "STREQUAL" "/path/to/install/llvm/bin/llvm-tblgen.exe"
      Unknown arguments specified

llvm-svn: 305159