Sema: Signatures of decls in local contexts always inherit availability.

Declarations nested in inlinable function bodies, such as `var` declarations,
should not introduce new `TypeRefinementContext` nodes that are restricted to
the deployment target. This bug was preventing availability checking from
noticing availability errors when the occurred in a `var` declaration inside of
a fragile function body:

```
@available(macOS 12.0, *)
public struct X {
  public init() {}
}

@_alwaysEmitIntoClient
public func x() {
  // This should be diagnosed, X is not always available
  let _ = X()
}
```

Resolves rdar://125564069

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -682,6 +682,11 @@ class TypeRefinementContextBuilder : private ASTWalker {
     if (isCurrentTRCContainedByDeploymentTarget())
       return false;
 
+    // A declaration inside of a local context always inherits the availability
+    // of the parent.
+    if (D->getDeclContext()->isLocalContext())
+      return false;
+
     // As a convenience, SPI decls and explicitly unavailable decls are
     // constrained to the deployment target. There's not much benefit to
     // checking these declarations at a lower availability version floor since

test/attr/attr_inlinable_available.swift

@@ -368,7 +368,7 @@ public func spiDeployedUseNoAvailable( // expected-note 3 {{add @available attri
 // using the minimum inlining target.
 //
 
-@inlinable public func inlinedUseNoAvailable( // expected-note 8 {{add @available attribute}}
+@inlinable public func inlinedUseNoAvailable( // expected-note 13 {{add @available attribute}}
   _: NoAvailable,
   _: BeforeInliningTarget,
   _: AtInliningTarget,
@@ -396,6 +396,28 @@ public func spiDeployedUseNoAvailable( // expected-note 3 {{add @available attri
   if #available(macOS 11, *) {
     _ = AfterDeploymentTarget()
   }
+
+  // Repeat everything with pattern binding decls instead of discard expressions.
+  defer {
+    let _ = AtDeploymentTarget() // expected-error {{'AtDeploymentTarget' is only available in macOS 10.15 or newer; clients of 'Test' may have a lower deployment target}} expected-note {{add 'if #available'}}
+    let _ = AfterDeploymentTarget() // expected-error {{'AfterDeploymentTarget' is only available in macOS 11 or newer}} expected-note {{add 'if #available'}}
+  }
+  let _ = NoAvailable()
+  let _ = BeforeInliningTarget()
+  let _ = AtInliningTarget()
+  let _ = BetweenTargets() // expected-error {{'BetweenTargets' is only available in macOS 10.14.5 or newer; clients of 'Test' may have a lower deployment target}} expected-note {{add 'if #available'}}
+  let _ = AtDeploymentTarget() // expected-error {{'AtDeploymentTarget' is only available in macOS 10.15 or newer; clients of 'Test' may have a lower deployment target}} expected-note {{add 'if #available'}}
+  let _ = AfterDeploymentTarget() // expected-error {{'AfterDeploymentTarget' is only available in macOS 11 or newer}} expected-note {{add 'if #available'}}
+
+  if #available(macOS 10.14.5, *) {
+    let _ = BetweenTargets()
+  }
+  if #available(macOS 10.15, *) {
+    let _ = AtDeploymentTarget()
+  }
+  if #available(macOS 11, *) {
+    let _ = AfterDeploymentTarget()
+  }
 }
 
 @available(macOS 10.9, *)