Move ASTGen to the Swift compiler; keep only CMake changes.

Author: zoecarver

Sources/ASTGen/ASTGen.swift

@@ -9,4 +9,3 @@
 add_subdirectory(SwiftSyntax)
 add_subdirectory(SwiftDiagnostics)
 add_subdirectory(SwiftParser)
-add_subdirectory(ASTGen)

Sources/ASTGen/CMakeLists.txt

@@ -93,6 +93,7 @@ extension Unicode.Scalar {
   }
 }
 
+#if swift(<5.7)
 extension UnsafeRawBufferPointer {
   /// Returns a typed buffer to the memory referenced by this buffer,
   /// assuming that the memory is already bound to the specified type.
@@ -121,6 +122,7 @@ extension UnsafeRawBufferPointer {
     return .init(start: s.assumingMemoryBound(to: T.self), count: n)
   }
 }
+#endif
 
 private var InfoTable: CharacterInfoTable = (
   // 0 NUL        1 SOH         2 STX         3 ETX

Sources/CMakeLists.txt

@@ -163,6 +163,7 @@ public class BumpPtrAllocator {
 
 // MARK: Compatibilty Shims
 
+#if swift(<5.7)
 extension UnsafeMutableRawPointer {
   /// Obtain the next pointer whose bit pattern is a multiple of alignment.
   ///
@@ -180,3 +181,4 @@ extension UnsafeMutableRawPointer {
     return .init(bitPattern: bits)!
   }
 }
+#endif