stdlib: Add some inlining attributes to help the optimizer getting the right inlining decisions.

This is more or less a workaround for some optimizations (mainly ARC opt) to avoid performance degradation with the upcoming inliner changes
In some situations it makes a big difference for ARC opt if a function is inlined or not, althought this shouldn't be the case.

stdlib/public/core/StringBuffer.swift

@@ -200,6 +200,7 @@ public struct _StringBuffer {
   /// - parameter bounds: Range of the substring that the caller tries
   ///   to extend.
   /// - parameter newUsedCount: The desired size of the substring.
+  @inline(__always)
   mutating func grow(
     oldBounds bounds: Range<UnsafePointer<_RawByte>>, newUsedCount: Int
   ) -> Bool {