[Macros] Allow keywords after # in freestanding macro expansions

Allow keywords after `#` in freestanding macro expansions

There is no reason why we shouldn’t allow keywords here.

I also thought about allowing keywords after `@` but things become tricky here for two reasons:
- In the parser, we parse a type after the `@`, which could start with a keyword itself (e.g. `any`). If we want to keep the parser logic to parse a type after `@` (which I think we should), then it becomes unclear what `@any T` should parse as.
- We allow a space between `@` and the type name. This makes it very hard for recovery to tell whether `@ struct` refers to an attribute with name `struct` or if the user forgot to write the attribute name after `@`.

Since almost all keywords are lowercase and attached member macros are usually spelled with an uppercase name, there are a lot fewer chances for clashes here, so I don’t think it’s worth allowing keywords after `@`.

https://github.com/apple/swift/issues/66444
rdar://110472060

lib/Parse/ParseExpr.cpp

@@ -2301,16 +2301,7 @@ ParserStatus Parser::parseFreestandingMacroExpansion(
 
   bool hasWhitespaceBeforeName = poundEndLoc != Tok.getLoc();
 
-  // Diagnose and parse keyword right after `#`.
-  if (Tok.isKeyword() && !hasWhitespaceBeforeName) {
-    diagnose(Tok, diag::keyword_cant_be_identifier, Tok.getText());
-    diagnose(Tok, diag::backticks_to_escape)
-        .fixItReplace(Tok.getLoc(), "`" + Tok.getText().str() + "`");
-
-    // Let 'parseDeclNameRef' to parse this as an identifier.
-    Tok.setKind(tok::identifier);
-  }
-  macroNameRef = parseDeclNameRef(macroNameLoc, diag, DeclNameOptions());
+  macroNameRef = parseDeclNameRef(macroNameLoc, diag, DeclNameFlag::AllowKeywords);
   if (!macroNameRef)
     return makeParserError();
 

test/Macros/macro_keywordname.swift

@@ -53,7 +53,7 @@ import MacroLib
 @freestanding(expression) public macro `class`() -> Int = #externalMacro(module: "MacroDefinition", type: "OneMacro")
 
 func test() {
-  let _: Int = #public() // expected-error {{keyword 'public' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}}
+  let _: Int = #public()
   let _: Int = #`public`()
   let _: Int = #escaped()
   let _: Int = #`class`()

test/Macros/macro_self.swift

@@ -9,11 +9,11 @@ func sync() {}
 macro Self() = #externalMacro(module: "MacroDefinition", type: "InvalidMacro")
 
 func testSelfAsFreestandingMacro() {
-  _ = #self // expected-error {{keyword 'self' cannot be used as an identifier here}} expected-note {{use backticks to escape it}}
+  _ = #self
 }
 
 func testCapitalSelfAsFreestandingMacro() {
-  _ = #Self // expected-error {{keyword 'Self' cannot be used as an identifier here}} expected-note {{use backticks to escape it}}
+  _ = #Self
 }
  
 func testSelfAsAttachedMacro() {

test/Parse/macro_decl.swift

@@ -51,6 +51,6 @@ public # someFunc // expected-error {{extraneous whitespace between '#' and macr
 
 struct S {
   # someFunc // expected-error {{extraneous whitespace between '#' and macro name is not permitted}} {{4-5=}}
-  #class // expected-error {{keyword 'class' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{4-9=`class`}} 
+  #class
   # struct Inner {} // expected-error {{expected a macro identifier for a pound literal declaration}} expected-error {{consecutive declarations on a line}}
 }

test/Parse/macro_expr.swift

@@ -44,7 +44,7 @@ do {
   _ = # macro() // expected-error {{extraneous whitespace between '#' and macro name is not permitted}} {{8-9=}}
 }
 do {
-  _ = #public() // expected-error {{keyword 'public' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{8-14=`public`}}
+  _ = #public()
 }
 do {
   _ = # public() // expected-error {{expected a macro identifier for a pound literal expression}}

test/decl/import/import.swift

@@ -35,9 +35,9 @@ import func Swift.print
 
 // rdar://14418336
 #import something_nonexistent
-// expected-error@-1 {{keyword 'import' cannot be used as an identifier here}} expected-note@-1 {{use backticks to escape it}}
-// expected-error@-2 {{no macro named 'import'}}
-// expected-error@-3 {{consecutive statements on a line}} expected-error@-3 {{cannot find 'something_nonexistent' in scope}}
+// expected-error@-1 {{no macro named 'import'}}
+// expected-error@-2 {{consecutive statements on a line}} 
+// expected-error@-3 {{cannot find 'something_nonexistent' in scope}}
 
 // Import specific decls
 import typealias Swift.Int