[Macros] Fix mangling scheme in the basic macro expansion context

Author: DougGregor

Sources/SwiftSyntaxMacros/BasicMacroExpansionContext.swift

@@ -122,11 +122,10 @@ extension BasicMacroExpansionContext: MacroExpansionContext {
     resultString += "fMu"
 
     // Mangle the index.
-    if uniqueIndex == 0 {
-      resultString += "_"
-    } else {
+    if uniqueIndex > 0 {
       resultString += "\(uniqueIndex - 1)"
     }
+    resultString += "_"
 
     return TokenSyntax(.identifier(resultString), presence: .present)
   }

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -732,7 +732,7 @@ final class MacroSystemTests: XCTestCase {
     let t2 = context.createUniqueName("mine")
     XCTAssertNotEqual(t1.description, t2.description)
     XCTAssertEqual(t1.description, "__macro_local_4minefMu_")
-    XCTAssertEqual(t2.description, "__macro_local_4minefMu0")
+    XCTAssertEqual(t2.description, "__macro_local_4minefMu0_")
   }
 
   func testContextIndependence() {