build-script: Build lldb with CMake on Darwin

build-script can already build lldb on Darwin using its Xcode project,
but it's useful to support the CMake build as well.

The CMake build allows incremental rebuilds with build-script. I expect
this to significantly cut down on iteration time.

Taking advantage of CMake also lets lldb piggyback on existing support
for sanitizers -- or exciting new build configurations we don't know
about yet -- without having to update the Xcode project file.

rdar://problem/36751944

utils/build_swift/driver_arguments.py

@@ -53,7 +53,8 @@ def _apply_default_arguments(args):
 
     # Build LLDB if any LLDB-related options were specified.
     if args.lldb_build_variant is not None or \
-       args.lldb_assertions is not None:
+       args.lldb_assertions is not None or \
+       args.lldb_build_with_xcode is not None:
         args.build_lldb = True
 
     # Set the default build variant.
@@ -75,6 +76,9 @@ def _apply_default_arguments(args):
     if args.lldb_build_variant is None:
         args.lldb_build_variant = args.build_variant
 
+    if args.lldb_build_with_xcode is None:
+        args.lldb_build_with_xcode = '1'
+
     if args.foundation_build_variant is None:
         args.foundation_build_variant = args.build_variant
 
@@ -568,6 +572,14 @@ def create_argument_parser():
            const='Debug',
            help='build the Debug variant of LLDB')
 
+    option('--lldb-build-with-xcode', store('lldb_build_with_xcode'),
+           const='1',
+           help='build LLDB using xcodebuild, if possible')
+
+    option('--lldb-build-with-cmake', store('lldb_build_with_xcode'),
+           const='0',
+           help='build LLDB using CMake')
+
     option('--debug-cmark', store('cmark_build_variant'),
            const='Debug',
            help='build the Debug variant of CommonMark')