Simplify assertion machinery in the standard library.

This change includes a number of simplifications that allow us to
eliminate the type checker hack that specifically tries
AssertString. Doing so provides a 25% speedup in the
test/stdlib/ArrayNew.swift test (which is type-checker bound).

The specific simplifications here:
  - User-level
  assert/precondition/preconditionalFailure/assertionFailer/fatalError
  always take an autoclosure producing a String, eliminating the need
  for the StaticString/AssertString dance.
  - Standard-library internal _precondition/_sanityCheck/etc. always
  take a StaticString. When we want to improve the diagnostics in the
  standard library, we can provide a separate overload or
  differently-named function.
  - Remove AssertString, AssertStringType, StaticStringType, which are
  no longer used or needed
  - Remove the AssertString hack from the compiler
  - Remove the "BooleanType" overloads of these functions, because
  their usefuless left when we stopped making optional types conform
  to BooleanType (sorry, should have been a separate patch).



Swift SVN r22139

lib/Sema/ConstraintSystem.cpp

@@ -284,14 +284,6 @@ getAlternativeLiteralTypes(KnownProtocolKind kind) {
   case KnownProtocolKind::StringInterpolationConvertible:
   case KnownProtocolKind::StringLiteralConvertible:
   case KnownProtocolKind::UnicodeScalarLiteralConvertible:
-    {
-      UnqualifiedLookup lookup(TC.Context.getIdentifier("AssertString"),
-                               DC->getModuleScopeContext(),
-                               nullptr);
-      if (auto typeDecl = lookup.getSingleTypeResult()) {
-        types.push_back(typeDecl->getDeclaredInterfaceType());
-      }
-    }
     break;
 
   case KnownProtocolKind::IntegerLiteralConvertible: