[build-script] Add an option to build-script called --lto that unifies the triggering of ltoing llvm/swift tools.

Also change the relevant build-presets to use this flag instead of touching
build-script-impl directly.

rdar://24717107

Author: gottesmm

utils/build-presets.ini

@@ -182,15 +182,15 @@ test-optimized=0
 mixin-preset=
     mixin_buildbot_tools_RA_stdlib_RDA
 
+lto
+
 dash-dash
 
 # Don't run host tests for iOS, tvOS and watchOS platforms to make the build
 # faster.
 skip-test-ios-host
 skip-test-tvos-host
 skip-test-watchos-host
-swift-enable-lto
-llvm-enable-lto
 
 #===------------------------------------------------------------------------===#
 # Incremental buildbots for Darwin OSes
@@ -415,9 +415,9 @@ skip-build-benchmarks
 mixin-preset=buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx
 build-subdir=buildbot_incremental
 
-dash-dash
+lto
 
-swift-enable-lto
+dash-dash
 
 [preset: buildbot_incremental,tools=RA,llvm-only]
 build-subdir=buildbot_incremental_llvmonly

utils/build-script

@@ -976,6 +976,16 @@ details of the setups of other systems or automated environments.""")
         default=False,
         const=True)
 
+    parser.add_argument(
+        "--lto",
+        help="use lto optimization on llvm/swift tools. This does not "
+             "imply using lto on the swift standard library or runtime",
+        metavar="BOOL",
+        nargs='?',
+        type=arguments.type.bool,
+        default=False,
+        const=True)
+
     parser.add_argument(
         # Explicitly unavailable options here.
         "--build-jobs",
@@ -1442,6 +1452,12 @@ details of the setups of other systems or automated environments.""")
                 pipes.quote(opt) for opt in args.extra_cmake_options)
         ]
 
+    if args.lto:
+        build_script_impl_args += [
+            "--llvm-enable-lto",
+            "--swift-enable-lto"
+        ]
+
     build_script_impl_args += args.build_script_impl_args
 
     if args.dry_run: