[ASTPrinter/Parse] Disambiguate accessor block in .swiftinterface

.swiftinterface sometimes prints a pattern binding initializer and the
accessor block. However the parser doesn't expect such constructs and
the disambiguation from trailing closures would be fragile. To make it
reliable, introduce a disambiguation marker `@_accessorBlock` .
`ASTPrinter` prints it right after `{` only if 1) The accessor block is
for a pattern binding declaration, 2) the decl has an initializer
printed, and 3) the non-observer accessor block is being printed. In the
parser, if the block after an initializer starts with
`{ @_accessorBlock`, it's always parsed as an accessor block instead of
a trailing closure.

rdar://140943107

test/ModuleInterface/var-with-init-and-accessors.swiftinterface

@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t/Test.swiftmodule -emit-module-interface-path %t/Test.swiftinterface -module-name Test %s
+// RUN: %target-swift-typecheck-module-from-interface(%t/Test.swiftinterface) -module-name Test
+
+// RUN: %FileCheck %s --check-prefix INTERFACE --input-file %t/Test.swiftinterface
+
+@frozen public struct Struct {
+    public var values: [Int] = .init() {
+        willSet {}
+    }
+}
+// INTERFACE: @frozen public struct Struct {
+// INTERFACE-LABEL: @_hasStorage public var values: [Swift.Int] = .init() { @_accessorBlock
+// INTERFACE-NEXT:     @_transparent get
+// INTERFACE-NEXT:     set
+// INTERFACE-NEXT:   }
+// INTERFACE: }