build: remove the requirement to checkout swift-syntax

In the case that a shared copy of swift-syntax is being used to build,
allow the sources to be unavailable. If the user specifies
`SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE`, that will be preferred,
otherwise, we fallback to the upstream git repository.

Author: compnerd

BuildSupport/SwiftSyntax/CMakeLists.txt

@@ -1,17 +1,14 @@
-SET(SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE ${CMAKE_SOURCE_DIR}/../swift-syntax)
-message(STATUS "swift-syntax path: ${SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE}")
-
 include(FetchContent)
 
-if(NOT EXISTS "${SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE}")
-  message(SEND_ERROR "swift-syntax is required to build SwiftPM. Please run update-checkout or specify SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE")
-  return()
-endif()
-
-# Build swift-syntax libraries with FetchContent.
-# set(CMAKE_Swift_COMPILER_TARGET ${SWIFT_HOST_TRIPLE})
 set(BUILD_SHARED_LIBS OFF)
 
-file(TO_CMAKE_PATH "${SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE}" swift_syntax_path)
-FetchContent_Declare(SwiftSyntax SOURCE_DIR "${swift_syntax_path}")
+if(DEFINED SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE)
+  file(TO_CMAKE_PATH "${SWIFTPM_PATH_TO_SWIFT_SYNTAX_SOURCE}" swift_syntax_path)
+  FetchContent_Declare(SwiftSyntax
+    SOURCE_DIR "${swift_syntax_path}")
+else()
+  FetchContent_Declare(SwiftSyntax
+    GIT_REPOSITORY https://github.com/apple/swift-syntax
+    GIT_TAG main)
+endif()
 FetchContent_MakeAvailable(SwiftSyntax)