Add Feature: NonescapableAccessorOnTrivial

To guard the new UnsafeMutablePointer.mutableSpan APIs.

This allows older compilers to ignore the new APIs. Otherwise, the type checker
will crash on the synthesized _read accessor for a non-Escapable type:

    error: cannot infer lifetime dependence on the '_read' accessor because 'self'
    is BitwiseCopyable, specify '@lifetime(borrow self)'

I don't know why the _read is synthesized in these cases, but apparently it's
always been that way.

Fixes: rdar://153773093 ([nonescapable] add a compiler feature to guard
~Escapable accessors when self is trivial)

(cherry picked from commit cc357f4f32)

test/ModuleInterface/Inputs/lifetime_dependence.swift

@@ -77,3 +77,21 @@ extension Container {
     }
   }
 }
+
+// Test feature guard: NonescapableAccessorOnTrivial
+extension UnsafeMutableBufferPointer where Element: ~Copyable {
+  public var span: Span<Element> {
+    @lifetime(borrow self)
+    @_alwaysEmitIntoClient
+    get {
+      unsafe Span(_unsafeElements: self)
+    }
+  }
+  public var mutableSpan: MutableSpan<Element> {
+    @lifetime(borrow self)
+    @_alwaysEmitIntoClient
+    get {
+      unsafe MutableSpan(_unsafeElements: self)
+    }
+  }
+}

test/ModuleInterface/lifetime_dependence_test.swift

@@ -2,6 +2,7 @@
 
 // RUN: %target-swift-frontend -swift-version 5 -enable-library-evolution -emit-module \
 // RUN:     -enable-experimental-feature LifetimeDependence \
+// RUN:     -suppress-warnings \
 // RUN:     -o %t/lifetime_dependence.swiftmodule \
 // RUN:     -emit-module-interface-path %t/lifetime_dependence.swiftinterface \
 // RUN:     %S/Inputs/lifetime_dependence.swift
@@ -41,3 +42,13 @@ import lifetime_dependence
 // CHECK: extension lifetime_dependence.Container {
 // CHECK-NEXT: #if compiler(>=5.3) && $NonescapableTypes && $LifetimeDependence
 // CHECK-NEXT:   public var storage: lifetime_dependence.BufferView {
+
+// CHECK-LABEL: extension Swift.UnsafeMutableBufferPointer where Element : ~Copyable {
+// CHECK:   #if compiler(>=5.3) && $LifetimeDependence
+// CHECK:   public var span: Swift.Span<Element> {
+// CHECK:     @lifetime(borrow self)
+// CHECK:     @_alwaysEmitIntoClient get {
+// CHECK:   #if compiler(>=5.3) && $LifetimeDependence && $NonescapableAccessorOnTrivial
+// CHECK:   public var mutableSpan: Swift.MutableSpan<Element> {
+// CHECK:     @lifetime(borrow self)
+// CHECK:     @_alwaysEmitIntoClient get {