[ASTGen] Support older host compilers

Author: DougGregor

lib/ASTGen/Sources/ASTGen/ASTGen.swift

@@ -72,7 +72,7 @@ struct ASTGenVisitor: SyntaxTransformVisitor {
 /// Generate AST nodes for all top-level entities in the given source file.
 @_cdecl("swift_ASTGen_buildTopLevelASTNodes")
 public func buildTopLevelASTNodes(
-  sourceFilePtr: UnsafeRawPointer,
+  sourceFilePtr: UnsafePointer<UInt8>,
   dc: UnsafeMutableRawPointer,
   ctx: UnsafeMutableRawPointer,
   outputContext: UnsafeMutableRawPointer,

lib/ASTGen/Sources/ASTGen/SourceFile.swift

@@ -40,7 +40,7 @@ public func parseSourceFile(
 /// Deallocate a parsed source file.
 @_cdecl("swift_ASTGen_destroySourceFile")
 public func destroySourceFile(
-  sourceFilePtr: UnsafeMutableRawPointer
+  sourceFilePtr: UnsafeMutablePointer<UInt8>
 ) {
   sourceFilePtr.withMemoryRebound(to: ExportedSourceFile.self, capacity: 1) { sourceFile in
     sourceFile.deinitialize(count: 1)