Don't always treat a closure following a variable declaration as an accessor block

Introduce some lookahead to make sure we actually have something that
looks like an accessor block when the `{` is on the next line, matching what
the C++ parser does.

Fixes swiftlang/swift#76089

Author: DougGregor

Sources/SwiftParser/Declarations.swift

@@ -1354,7 +1354,8 @@ extension Parser {
         }
 
         let accessors: RawAccessorBlockSyntax?
-        if self.at(.leftBrace)
+        if (self.at(.leftBrace)
+          && (!self.currentToken.isAtStartOfLine || self.withLookahead({ $0.atStartOfGetSetAccessor() })))
           || (inMemberDeclList && self.at(anyIn: AccessorDeclSyntax.AccessorSpecifierOptions.self) != nil
             && !self.at(.keyword(.`init`)))
         {

Tests/SwiftParserTest/ExpressionTests.swift

@@ -58,6 +58,18 @@ final class ExpressionTests: ParserTestCase {
       }
       """
     )
+
+    assertParse(
+      """
+      func f(x:[Void])
+      {
+        var y:[[Void]] = x.map { [$0] }
+        {
+          $0.reserveCapacity(1)
+        } (&y[0])
+      }
+      """
+    )
   }
 
   func testTrailingClosures() {