stdlib: remove some inlinable annotations from Substring's and String's subscript

Inlining those operations does not really give any benefit.
Those operations are complex and most likely, cannot be folded into simpler patterns after inlining, anyway.
Reduces some code size for cases where those functions were inlined (due to a not perfect inlining heuristics).

Author: eeckstein

stdlib/public/core/Substring.swift

@@ -200,9 +200,8 @@ extension Substring: StringProtocol {
     return _slice.distance(from: start, to: end)
   }
 
-  @inlinable
   public subscript(i: Index) -> Character {
-    @inline(__always) get { return _slice[i] }
+    get { return _slice[i] }
   }
 
   public mutating func replaceSubrange<C>(
@@ -322,7 +321,6 @@ extension Substring: CustomDebugStringConvertible {
 }
 
 extension Substring: LosslessStringConvertible {
-  @inlinable
   public init(_ content: String) {
     self = content[...]
   }
@@ -800,7 +798,6 @@ extension Substring: ExpressibleByStringLiteral {
 
 // String/Substring Slicing
 extension String {
-  @inlinable
   @available(swift, introduced: 4)
   public subscript(r: Range<Index>) -> Substring {
     _boundsCheck(r)
@@ -809,7 +806,6 @@ extension String {
 }
 
 extension Substring {
-  @inlinable
   @available(swift, introduced: 4)
   public subscript(r: Range<Index>) -> Substring {
     return Substring(_slice[r])