Fix misinterpretation of CMake rule found by a CMake warning (related to CMP0054).

lldb sets the variable SHARED_LIBRARY to 1, which breaks this conditional,
because older versions of CMake interpret

  if ("${t}" STREQUAL "SHARED_LIBRARY")

as meaning

  if ("${t}" STREQUAL "1")

in this case. Change the conditional so it does the right thing with both old
and new CMakes.

llvm-svn: 218542