[SwiftCompiler] Add DiagnosticEngine bridging

* 'SourceLoc' and 'CharSourceRange' bridging in Basic
* New 'AST' bridging. 'DiagID', 'DiagnosticArgument', 'DiagnosticFixIt',
  and 'DiagnosticEngine'

include/swift/module.modulemap

@@ -4,6 +4,11 @@ module BasicBridging {
   export *
 }
 
+module ASTBridging {
+  header "AST/ASTBridging.h"
+  export *
+}
+
 module SILBridging {
   header "SIL/SILBridging.h"
   export *