swift234

[lto] Do not pass in the LLVM_ENABLE_LTO flag to Swift's cmake invocation.

*NOTE* We are still passing this variable into LLVM. Just not into Swift anymore.

I originally passed in the variable -DLLVM_ENABLE_LTO=YES to Swift's cmake to
ensure that the Swift unittests were compiled with lto. That was the wrong
fix.

The reason why is that if -DLLVM_ENABLE_LTO is set to YES then HandleLLVMOptions
in llvm will append -flto to both CMAKE_C_FLAGS and CMAKE_CXX_FLAGS implying
that -flto can not be disabled selectively. This ability to disable flto
selectively is something that we need in order to lto host libraries, but not
lto target libraries.

Thus in this commit, we no longer pass in -DLLVM_ENABLE_LTO=YES to swift's cmake
invocation. Instead we apply the lto flag to the unittest target ourselves in
add_swift_unittest.

rdar://24717107