Implement an unsafe expression to cover uses of unsafe constructs

Introduce an `unsafe` expression akin to `try` and `await` that notes
that there are unsafe constructs in the expression to the right-hand
side. Extend the effects checker to also check for unsafety along with
throwing and async operations. This will result in diagnostics like
the following:

    10 |   func sum() -> Int {
    11 |     withUnsafeBufferPointer { buffer in
    12 |       let value = buffer[0]
       |                   |     `- note: reference to unsafe subscript 'subscript(_:)'
       |                   |- warning: expression uses unsafe constructs but is not marked with 'unsafe'
       |                   `- note: reference to parameter 'buffer' involves unsafe type 'UnsafeBufferPointer<Int>'
    13 |       tryWithP(X())
    14 |       return fastAdd(buffer.baseAddress, buffer.count)

These will come with a Fix-It that inserts `unsafe` into the proper
place. There's also a warning that appears when `unsafe` doesn't cover
any unsafe code, making it easier to clean up extraneous `unsafe`.

This approach requires that `@unsafe` be present on any declaration
that involves unsafe constructs within its signature. Outside of the
signature, the `unsafe` expression is used to identify unsafe code.

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1707,12 +1707,12 @@ private:
         auto SR = E->getSourceRange();
         if (SR.isValid() && SM.rangeContainsTokenLoc(SR, TargetLoc) &&
             !checkCallExpr(E) && !EnclosingCallAndArg.first) {
-          if (!isa<TryExpr>(E) && !isa<AwaitExpr>(E) &&
+          if (!isa<TryExpr>(E) && !isa<AwaitExpr>(E) && !isa<UnsafeExpr>(E) &&
               !isa<PrefixUnaryExpr>(E)) {
             // We don't want to expand to trailing closures if the call is
             // nested inside another expression that has closing characters,
             // like a `)` for a function call. This is not the case for
-            // `try`, `await` and prefix operator applications.
+            // `try`, `await`, `unsafe` and prefix operator applications.
             OuterExpr = E;
           }
         }