[Parse] NFC: Allow sub-lexers to disable diagnostics

And `const` qualify a couple of params/methods.

Author: hamishknight

include/swift/AST/DiagnosticEngine.h

@@ -1390,7 +1390,7 @@ namespace swift {
     DiagnosticEngine &getDiags() { return QueueEngine; }
 
     /// Retrieve the underlying engine which will receive the diagnostics.
-    DiagnosticEngine &getUnderlyingDiags() { return UnderlyingEngine; }
+    DiagnosticEngine &getUnderlyingDiags() const { return UnderlyingEngine; }
 
     /// Clear this queue and erase all diagnostics recorded.
     void clear() {

include/swift/Parse/Lexer.h

@@ -180,7 +180,7 @@ class Lexer {
   /// Retrieve the underlying diagnostic engine we emit diagnostics to. Note
   /// this should only be used for diagnostics not concerned with the current
   /// token.
-  DiagnosticEngine *getUnderlyingDiags() {
+  DiagnosticEngine *getUnderlyingDiags() const {
     return DiagQueue ? &DiagQueue->getUnderlyingDiags() : nullptr;
   }
 
@@ -218,7 +218,10 @@ class Lexer {
   /// \param Parent the parent lexer that scans the whole buffer
   /// \param BeginState start of the subrange
   /// \param EndState end of the subrange
-  Lexer(Lexer &Parent, State BeginState, State EndState);
+  /// \param EnableDiagnostics Whether to inherit the diagnostic engine of
+  /// \p Parent. If \c false, diagnostics will be disabled.
+  Lexer(const Lexer &Parent, State BeginState, State EndState,
+        bool EnableDiagnostics = true);
 
   /// Returns true if this lexer will produce a code completion token.
   bool isCodeCompletion() const {

lib/Parse/Lexer.cpp

@@ -246,9 +246,11 @@ Lexer::Lexer(const LangOptions &Options, const SourceManager &SourceMgr,
   initialize(Offset, EndOffset);
 }
 
-Lexer::Lexer(Lexer &Parent, State BeginState, State EndState)
+Lexer::Lexer(const Lexer &Parent, State BeginState, State EndState,
+             bool EnableDiagnostics)
     : Lexer(PrincipalTag(), Parent.LangOpts, Parent.SourceMgr, Parent.BufferID,
-            Parent.getUnderlyingDiags(), Parent.LexMode,
+            EnableDiagnostics ? Parent.getUnderlyingDiags() : nullptr,
+            Parent.LexMode,
             Parent.IsHashbangAllowed
                 ? HashbangMode::Allowed
                 : HashbangMode::Disallowed,