[6.2][StrictMemorySafety] Check the safety of return types of calls

Explanation: There were some scenarios where we could call an unsafe
function without marking the expression as unsafe. These affect mostly
cases where the function's result is passed to another function or
returned. This PR makes sure we always flag functions with unsafe return
types, even if their result is not stored anywhere for later use.
Issues: rdar://157237301
Original PRs: #83520
Risk: Low, worst case scenario the user has to add redundant unsafe
keywords in strict memory safe mode.
Testing: Added a compiler test.
Reviewers: @DougGregor

stdlib/public/core/StringObject.swift

@@ -973,7 +973,7 @@ extension _StringObject {
 #if _runtime(_ObjC)
     if largeFastIsConstantCocoa {
       return unsafe withCocoaObject {
-        _getNSCFConstantStringContentsPointer($0)
+        unsafe _getNSCFConstantStringContentsPointer($0)
       }
     }
     if largeIsCocoa {
@@ -989,7 +989,7 @@ extension _StringObject {
   internal var sharedUTF8: UnsafeBufferPointer<UInt8> {
     @_effects(releasenone) @inline(never) get {
       _internalInvariant(largeFastIsShared)
-      let start = self.getSharedUTF8Start()
+      let start = unsafe self.getSharedUTF8Start()
       return unsafe UnsafeBufferPointer(start: start, count: largeCount)
     }
   }