stdlib: Address StrictMemorySafety warnings in String related code.

Author: tshortli

stdlib/public/core/StringCreate.swift

@@ -383,7 +383,7 @@ extension String {
     }
 
     let storage = unsafe buffer.baseAddress.map {
-      __SharedStringStorage(
+      unsafe __SharedStringStorage(
         _mortal: $0,
         countAndFlags: _StringObject.CountAndFlags(
           count: buffer.startIndex.distance(to: written),

stdlib/public/core/StringGuts.swift

@@ -236,7 +236,7 @@ extension _StringGuts {
     _ body: (UnsafePointer<Int8>) throws -> Result
   ) rethrows -> Result {
     _internalInvariant(!_object.isFastZeroTerminated)
-    return try String(self).utf8CString.withUnsafeBufferPointer {
+    return try unsafe String(self).utf8CString.withUnsafeBufferPointer {
       let ptr = unsafe $0.baseAddress._unsafelyUnwrappedUnchecked
       return try unsafe body(ptr)
     }

stdlib/public/core/StringObject.swift

@@ -982,7 +982,7 @@ extension _StringObject {
       }
     }
 #endif
-    return unsafe withSharedStorage { $0.start }
+    return unsafe withSharedStorage { unsafe $0.start }
   }
 
   @usableFromInline

stdlib/public/core/StringStorage.swift

@@ -699,7 +699,7 @@ final internal class __SharedStringStorage
     countAndFlags: _StringObject.CountAndFlags
   ) {
     self._owner = nil
-    self.start = unsafe ptr
+    unsafe self.start = ptr
     self.immortal = true
 #if _pointerBitWidth(_64)
     self._countAndFlags = countAndFlags
@@ -728,7 +728,7 @@ final internal class __SharedStringStorage
   ) {
     // ptr *must* be the start of an allocation
     self._owner = nil
-    self.start = unsafe ptr
+    unsafe self.start = ptr
     self.immortal = false
 #if _pointerBitWidth(_64)
     self._countAndFlags = countAndFlags

stdlib/public/core/StringStorageBridge.swift

@@ -312,7 +312,7 @@ extension __SharedStringStorage {
   @objc(UTF8String)
   @_effects(readonly)
   final internal func _utf8String() -> UnsafePointer<UInt8>? {
-    return start
+    return unsafe start
   }
 
   @objc(cStringUsingEncoding:)

stdlib/public/core/StringUTF8View.swift

@@ -278,7 +278,7 @@ extension String {
     @_effects(readonly) @_semantics("string.getUTF8CString")
     get {
       if _fastPath(_guts.isFastUTF8) {
-        var result = unsafe _guts.withFastCChar { ContiguousArray($0) }
+        var result = unsafe _guts.withFastCChar { unsafe ContiguousArray($0) }
         result.append(0)
         return result
       }