[Macros] Adjust macro declaration syntax to match the latest pitch.

Instead of macro definitions always being defined via
`= ModuleName.TypeName`, parse the more general `= expression`, which
is already captured by the `InitializerClause` grammar production.
Additionally, make the definition optional, which accounts for
built-in macros.

Author: DougGregor

Sources/SwiftParser/Declarations.swift

@@ -2039,18 +2039,16 @@ extension Parser {
       signature = .functionLike(self.parseFunctionSignature())
     }
 
-    // External macro name
-    let (unexpectedBeforeEqual, equal) = self.expect(.equal)
-    let (unexpectedBeforeModuleName, moduleName) = self.expectIdentifier()
-    let (unexpectedBeforePeriod, period) = self.expect(.period)
-    let (unexpectedBeforeMacroTypeName, macroTypeName) = self.expectIdentifier()
-
-    let externalMacroName = RawExternalMacroNameSyntax(
-      unexpectedBeforeModuleName, moduleName: moduleName,
-      unexpectedBeforePeriod, period: period,
-      unexpectedBeforeMacroTypeName, macroTypeName: macroTypeName,
-      arena: self.arena
-    )
+    // Initializer, if any.
+    let definition: RawInitializerClauseSyntax?
+    if let equal = self.consume(if: .equal) {
+      let expr = self.parseExpression()
+      definition = RawInitializerClauseSyntax(
+        equal: equal, value: expr, arena: self.arena
+      )
+    } else {
+      definition = nil
+    }
 
     // Parse a 'where' clause if present.
     let whereClause: RawGenericWhereClauseSyntax?
@@ -2065,8 +2063,7 @@ extension Parser {
       unexpectedBeforeIntroducerKeyword, macroKeyword: introducerKeyword,
       unexpectedBeforeName, identifier: name,
       genericParameterClause: genericParams,
-      signature: signature, unexpectedBeforeEqual, equal: equal,
-      externalName: externalMacroName,
+      signature: signature, definition: definition,
       genericWhereClause: whereClause,
       arena: self.arena
     )

Tests/SwiftParserTest/DeclarationTests.swift

@@ -1321,15 +1321,14 @@ final class DeclarationTests: XCTestCase {
       macro m2(_: Int) = A.M2
       macro m3(a b: Int) -> Int = A.M3
       macro m4<T>: T = A.M4 where T.Assoc: P
-      macro m4<T: P>(_: T) = A.M4
+      macro m5<T: P>(_: T)
       """)
 
     AssertParse("""
-      macro m1 1️⃣= A2️⃣
+      macro m1 1️⃣= A
       """,
       diagnostics: [
         DiagnosticSpec(locationMarker: "1️⃣", message: "expected parameter clause in function signature"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected '.' and macro type name in external macro name"),
       ]
     )
   }

gyb_syntax_support/DeclNodes.py

@@ -933,19 +933,11 @@
                        Child('FunctionLike', kind='FunctionSignature', name_for_diagnostics="macro signature"),
                        Child('ValueLike', kind='TypeAnnotation', name_for_diagnostics="macro signature"),
                    ]),
-             Child('Equal', kind='EqualToken'),
-             Child('ExternalName', kind='ExternalMacroName', name_for_diagnostics='external macro name', is_optional = True),
+             Child('Definition', kind='InitializerClause', name_for_diagnostics='macro definition', is_optional=True),
              Child('GenericWhereClause', kind='GenericWhereClause', name_for_diagnostics='generic where clause',
                    is_optional=True),
          ]),
 
-    Node('ExternalMacroName', name_for_diagnostics='external macro name', kind='Syntax',
-         children=[
-             Child('ModuleName', kind='IdentifierToken', name_for_diagnostics='module name'),
-             Child('Period', kind='PeriodToken'),
-             Child('MacroTypeName', kind='IdentifierToken', name_for_diagnostics='macro type name'),
-         ]),
-
     # e.g., "#embed("filename.txt")"
     Node('MacroExpansionDecl',
          name_for_diagnostics="pound literal declaration", kind='Decl',