Remove inaccurate comments

Author: beccadax

stdlib/public/core/Mirror.swift

@@ -547,7 +547,7 @@ extension String {
   ///
   ///     print(String(describing: p))
   ///     // Prints "(21, 30)"
-  @inlinable // FIXME(sil-serialize-all)
+  @inlinable
   public init<Subject: CustomStringConvertible>(describing instance: Subject) {
     self = instance.description
   }
@@ -591,7 +591,7 @@ extension String {
   ///
   ///     print(String(describing: p))
   ///     // Prints "(21, 30)"
-  @inlinable // FIXME(sil-serialize-all)
+  @inlinable
   public init<Subject: TextOutputStreamable>(describing instance: Subject) {
     self.init()
     instance.write(to: &self)
@@ -636,7 +636,7 @@ extension String {
   ///
   ///     print(String(describing: p))
   ///     // Prints "(21, 30)"
-  @inlinable // FIXME(sil-serialize-all)
+  @inlinable
   public init<Subject>(describing instance: Subject)
     where Subject: CustomStringConvertible & TextOutputStreamable
   {

stdlib/public/core/StringGuts.swift

@@ -919,7 +919,7 @@ extension _StringGuts {
     reserveCapacitySlow(capacity)
   }
 
-  @usableFromInline // @testable
+  @usableFromInline
   mutating func reserveCapacitySlow(_ capacity: Int) {
     if _fastPath(_isUniqueNative()) {
       if _fastPath(_object.nativeRawStorage.capacity >= capacity) {
@@ -1001,7 +1001,7 @@ extension _StringGuts {
     _appendSlow(other)
   }
 
-  @usableFromInline // @testable
+  @usableFromInline
   internal
   mutating func _appendSlow(_ other: _StringGuts) {
     if _slowPath(other._isOpaque) {