[Type checker] Don't overflow array bounds with #fileLiteral *ahem*.

Fixes rdar://problem/26586984.

Author: DougGregor

lib/Sema/ConstraintSystem.cpp

@@ -249,6 +249,7 @@ getAlternativeLiteralTypes(KnownProtocolKind kind) {
   case KnownProtocolKind::ExpressibleByImageLiteral: index = 11; break;
   case KnownProtocolKind::ExpressibleByFileReferenceLiteral: index = 12; break;
   }
+  static_assert(NumAlternativeLiteralTypes == 13, "Wrong # of literal types");
 
   // If we already looked for alternative literal types, return those results.
   if (AlternativeLiteralTypes[index])

lib/Sema/ConstraintSystem.h

@@ -925,7 +925,8 @@ class ConstraintSystem {
     MemberLookups;
 
   /// Cached sets of "alternative" literal types.
-  Optional<ArrayRef<Type>> AlternativeLiteralTypes[12];
+  static const unsigned NumAlternativeLiteralTypes = 13;
+  Optional<ArrayRef<Type>> AlternativeLiteralTypes[NumAlternativeLiteralTypes];
 
   /// \brief Folding set containing all of the locators used in this
   /// constraint system.

test/Sema/object_literals_osx.swift

@@ -22,3 +22,5 @@ struct Path: _ExpressibleByFileReferenceLiteral {
 
 let p1: Path = #fileLiteral(resourceName: "what.txt")
 let p2 = #fileLiteral(resourceName: "what.txt") // expected-error{{could not infer type of file reference literal}} expected-note{{import Foundation to use 'URL' as the default file reference literal type}}
+
+let text = #fileLiteral(resourceName: "TextFile.txt").relativeString! // expected-error{{type of expression is ambiguous without more context}}