[stdlib] Add an unsafe U[M]BP initializer to work around some inliner test failures

stdlib/public/core/SmallString.swift

@@ -208,7 +208,7 @@ extension _SmallString {
         // Restore the memory type of self._storage
         _ = rawPtr.bindMemory(to: RawBitPattern.self, capacity: 1)
       }
-      return try f(UnsafeBufferPointer(start: ptr, count: count))
+      return try f(UnsafeBufferPointer(_uncheckedStart: ptr, count: count))
     }
   }
 

stdlib/public/core/StringObject.swift

@@ -902,7 +902,7 @@ extension _StringObject {
       return sharedUTF8
     }
     return UnsafeBufferPointer(
-      start: self.nativeUTF8Start, count: self.largeCount)
+      _uncheckedStart: self.nativeUTF8Start, count: self.largeCount)
   }
 
   // Whether the object stored can be bridged directly as a NSString

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -394,6 +394,15 @@ extension Unsafe${Mutable}BufferPointer: ${Mutable}Collection, RandomAccessColle
 }
 
 extension Unsafe${Mutable}BufferPointer {
+  @_alwaysEmitIntoClient
+  internal init(
+    @_nonEphemeral _uncheckedStart start: Unsafe${Mutable}Pointer<Element>?,
+    count: Int
+  ) {
+    _position = start
+    self.count = count
+  }
+
   /// Creates a new buffer pointer over the specified number of contiguous
   /// instances beginning at the given pointer.
   ///
@@ -413,8 +422,7 @@ extension Unsafe${Mutable}BufferPointer {
     _debugPrecondition(
       count == 0 || start != nil,
       "Unsafe${Mutable}BufferPointer has a nil start and nonzero count")
-    _position = start
-    self.count = _assumeNonNegative(count)
+    self.init(_uncheckedStart: start, count: _assumeNonNegative(count))
   }
 
   @inlinable // unsafe-performance