[Build] Work around build-scripts single-root requirement.

Replace the correct Swift -> EarlySwiftSyntax dependency with an
EarlySwiftDriver -> EarlySwiftSyntax dependency, to keep a linear
dependency order in this part of the build graph.

Author: DougGregor

utils/swift_build_support/swift_build_support/products/earlyswiftdriver.py

@@ -12,6 +12,7 @@
 
 import os
 
+from . import earlyswiftsyntax
 from . import product
 from .. import shell
 from .. import toolchain
@@ -57,7 +58,11 @@ def should_build(self, host_target):
 
     @classmethod
     def get_dependencies(cls):
-        return []
+        # FIXME: This isn't a real dependency, but is necessary to linearize the
+        # dependency graph from Swift to EarlySwiftSyntax. If we properly
+        # express the dependency from Swift -> EarlySwiftSyntax, build_graph.py
+        # asserts that there are multiple roots to the graph.
+        return [earlyswiftsyntax.EarlySwiftSyntax]
 
     def should_clean(self, host_target):
         return self.args.clean_early_swift_driver

utils/swift_build_support/swift_build_support/products/swift.py

@@ -14,7 +14,6 @@
 
 from . import cmark
 from . import earlyswiftdriver
-from . import earlyswiftsyntax
 from . import libcxx
 from . import llvm
 from . import product
@@ -214,6 +213,5 @@ def _swift_tools_ld64_lto_codegen_only_for_supporting_targets(self):
     def get_dependencies(cls):
         return [cmark.CMark,
                 earlyswiftdriver.EarlySwiftDriver,
-                earlyswiftsyntax.EarlySwiftSyntax,
                 llvm.LLVM,
                 libcxx.LibCXX]