[shims] Re-add Foundation shims removed in swiftlang#28918

The current version of the Foundation overlay doesn’t use these, but we should still keep them in case a toolchain snapshot build picks up on overlay module from one of the SDKs in Xcode 11.

rdar://62339802

Author: lorentey

stdlib/public/SwiftShims/FoundationOverlayShims.h

@@ -34,6 +34,44 @@
 #import "NSTimeZoneShims.h"
 #import "NSUndoManagerShims.h"
 
+typedef struct {
+    void *_Nonnull memory;
+    size_t capacity;
+    _Bool onStack;
+} _ConditionalAllocationBuffer;
+
+static inline _Bool _resizeConditionalAllocationBuffer(_ConditionalAllocationBuffer *_Nonnull buffer, size_t amt) {
+    size_t amount = malloc_good_size(amt);
+    if (amount <= buffer->capacity) { return true; }
+    void *newMemory;
+    if (buffer->onStack) {
+        newMemory = malloc(amount);
+        if (newMemory == NULL) { return false; }
+        memcpy(newMemory, buffer->memory, buffer->capacity);
+        buffer->onStack = false;
+    } else {
+        newMemory = realloc(buffer->memory, amount);
+        if (newMemory == NULL) { return false; }
+    }
+    if (newMemory == NULL) { return false; }
+    buffer->memory = newMemory;
+    buffer->capacity = amount;
+    return true;
+}
+
+static inline _Bool _withStackOrHeapBuffer(size_t amount, void (__attribute__((noescape)) ^ _Nonnull applier)(_ConditionalAllocationBuffer *_Nonnull)) {
+    _ConditionalAllocationBuffer buffer;
+    buffer.capacity = malloc_good_size(amount);
+    buffer.onStack = (pthread_main_np() != 0 ? buffer.capacity < 2048 : buffer.capacity < 512);
+    buffer.memory = buffer.onStack ? alloca(buffer.capacity) : malloc(buffer.capacity);
+    if (buffer.memory == NULL) { return false; }
+    applier(&buffer);
+    if (!buffer.onStack) {
+        free(buffer.memory);
+    }
+    return true;
+}
+
 @protocol _NSKVOCompatibilityShim <NSObject>
 + (void)_noteProcessHasUsedKVOSwiftOverlay;
 @end