[NFC] Get rid of #include Lexer.h from ASTContext.cpp (swiftlang#27714)

The only other header that depends on Lexer.h is Sema/TypeChecker.h,
so it is safe to get rid of the #include from AST cpp files if it is
not being used without worrying about accidental transitive #includes.

Author: varungandhi-apple

lib/AST/ASTContext.cpp

@@ -47,7 +47,6 @@
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
 #include "swift/Basic/StringExtras.h"
-#include "swift/Parse/Lexer.h" // bad dependency
 #include "swift/Syntax/References.h"
 #include "swift/Syntax/SyntaxArena.h"
 #include "swift/Strings.h"