[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

Author: gottesmm

cmake/modules/AddSwiftUnittests.cmake

@@ -10,6 +10,16 @@ function(add_swift_unittest test_dirname)
   # function defined by AddLLVM.cmake.
   add_unittest(SwiftUnitTests ${test_dirname} ${ARGN})
 
+  # TODO: _add_variant_c_compile_link_flags and these tests should share some
+  # sort of logic.
+  #
+  # *NOTE* The unittests are never built for the target, so we always enable LTO
+  # *if we are asked to.
+  if (SWIFT_TOOLS_ENABLE_LTO)
+    set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY COMPILE_FLAGS " -flto ")
+    set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY LINK_FLAGS " -flto ")
+  endif()
+
   if(SWIFT_BUILT_STANDALONE AND NOT "${CMAKE_CFG_INTDIR}" STREQUAL ".")
     # Replace target references with full paths, so that we use LLVM's
     # build configuration rather than Swift's.

utils/build-script-impl

@@ -588,14 +588,6 @@ function set_build_options_for_host() {
                     )
                 fi
 
-                # We need to also pass in this flag so that parts of LLVM's
-                # build system that we use (for instance when compiling
-                # unittests) do not use too many threads when we are using lto.
-                if [[ $(true_false "${LLVM_ENABLE_LTO}") == "TRUE" ]]; then
-                    swift_cmake_options+=(
-                        "-DLLVM_PARALLEL_LINK_JOBS=${LLVM_NUM_PARALLEL_LTO_LINK_JOBS}"
-                    )
-                fi
                 swift_cmake_options+=(
                     "-DSWIFT_PARALLEL_LINK_JOBS=${SWIFT_TOOLS_NUM_PARALLEL_LTO_LINK_JOBS}"
                 )
@@ -1864,7 +1856,6 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DCMAKE_CXX_FLAGS="$(swift_c_flags ${host})"
                     -DCMAKE_BUILD_TYPE:STRING="${SWIFT_BUILD_TYPE}"
                     -DLLVM_ENABLE_ASSERTIONS:BOOL=$(true_false "${SWIFT_ENABLE_ASSERTIONS}")
-                    -DLLVM_ENABLE_LTO=$(true_false "${LLVM_ENABLE_LTO}")
                     -DSWIFT_ANALYZE_CODE_COVERAGE:STRING=$(toupper "${SWIFT_ANALYZE_CODE_COVERAGE}")
                     -DSWIFT_STDLIB_BUILD_TYPE:STRING="${SWIFT_STDLIB_BUILD_TYPE}"
                     -DSWIFT_STDLIB_ASSERTIONS:BOOL=$(true_false "${SWIFT_STDLIB_ENABLE_ASSERTIONS}")