[lto] Use -DLLVM_ENABLE_LTO to enable llvm lto instead of -DCMAKE_{C,CXX}_FLAGS

This not only is the more canonical way to enable lto in LLVM but in addition
without this change, the swift compiler unittests will not build with LTO
resulting in funky LTO errors like rdar://25968091.

rdar://24717107

Author: gottesmm

utils/build-script-impl

@@ -531,10 +531,6 @@ function set_deployment_target_based_options() {
                 fi
 
                 llvm_cmake_options+=(
-                    "-DCMAKE_C_FLAGS=-O2 -flto -gline-tables-only -fno-stack-protector "
-                    "-DCMAKE_CXX_FLAGS=-O2 -flto -gline-tables-only -fno-stack-protector "
-                    "-DCMAKE_C_FLAGS_RELWITHDEBINFO=-O2 -flto -gline-tables-only -fno-stack-protector "
-                    "-DCMAKE_CXX_FLAGS_RELWITHDEBINFO=-O2 -flto -gline-tables-only -fno-stack-protector "
                     "-DLLVM_PARALLEL_LINK_JOBS=$(num_llvm_parallel_lto_link_jobs)"
                 )
             fi
@@ -1239,6 +1235,9 @@ function llvm_c_flags() {
     if [[ $(is_cmake_release_build_type "${LLVM_BUILD_TYPE}") ]] ; then
         echo -n " -fno-stack-protector"
     fi
+    if [[ $(true_false "${LLVM_ENABLE_LTO}") ]] ; then
+        echo -n " -gline-tables-only"
+    fi
 }
 
 function cmark_c_flags() {
@@ -1527,6 +1526,7 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_TOOLS_DEPLOYMENT_TARGETS[@]}"
                     -DLLVM_TARGETS_TO_BUILD="${LLVM_TARGETS_TO_BUILD}"
                     -DLLVM_INCLUDE_TESTS:BOOL=$(true_false "${SOURCE_TREE_INCLUDES_TESTS}")
                     -DLLVM_INCLUDE_DOCS:BOOL=TRUE
+                    -DLLVM_ENABLE_LTO=$(true_false "${LLVM_ENABLE_LTO}")
                     "${llvm_cmake_options[@]}"
                 )
                 if [[ $(is_cross_tools_deployment_target ${deployment_target}) ]] ; then
@@ -1619,6 +1619,7 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_TOOLS_DEPLOYMENT_TARGETS[@]}"
                     -DCMAKE_CXX_FLAGS="$(swift_c_flags ${deployment_target})"
                     -DCMAKE_BUILD_TYPE:STRING="${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}")