[CMake] don't build libLTO when LLVM_ENABLE_PIC is OFF

When cmake is run with -DLLVM_ENABLE_PIC=OFF, build fails while
linking shared library libLTO.so, because its dependencies are built
with -fno-PIC. More details here: https://llvm.org/bugs/show_bug.cgi?id=26484.
This diff reverts r252652 (git 9fd4377ddb83aee3c049dc8757e7771edbb8ee71),
which removed check NOT LLVM_ENABLE_PIC before disabling build for libLTO.so.

Patch by Igor Sugak!

Differential Revision: http://reviews.llvm.org/D17049

llvm-svn: 260703