[SE-0458] Disambiguate existing uses of unsafe better

Only treat `unsafe` as starting an unsafe expression if the following token
is on the same line and isn't a `)`. The former is in line with how we
handle other modifier expressions like this, and the latter is because of
the archaic `@MainActor(unsafe)`.

Author: DougGregor

Sources/SwiftParser/Expressions.swift

@@ -445,6 +445,10 @@ extension Parser {
         )
       )
     case (.unsafe, let handle)?:
+      if self.peek().isAtStartOfLine || self.peek(isAt: .rightParen) {
+        break EXPR_PREFIX
+      }
+
       let unsafeTok = self.eat(handle)
       let sub = self.parseSequenceExpressionElement(
         flavor: flavor,

Tests/SwiftParserTest/ExpressionTests.swift

@@ -2200,24 +2200,11 @@ final class StatementExpressionTests: ParserTestCase {
     assertParse(
       """
       func f() {
-        let x = unsafe1️⃣ y
+        let x = unsafe
+        y
       }
       """,
-      diagnostics: [
-        DiagnosticSpec(
-          message: "consecutive statements on a line must be separated by newline or ';'",
-          fixIts: [
-            "insert newline",
-            "insert ';'",
-          ]
-        )
-      ],
-      fixedSource: """
-        func f() {
-          let x = unsafe
-          y
-        }
-        """
+      substructure: DeclReferenceExprSyntax(baseName: .identifier("unsafe"))
     )
   }