[parser] Make sure the isInputIncomplete() function works as expected when there is an empty string interpolation segment

Previously it was erroneously treating such invalid interpolation segment as 'incomplete',
even though additional user input, will not 'complete' it.

rdar://28498239

Author: akyrtzi

include/swift/Parse/Parser.h

@@ -192,7 +192,10 @@ class Parser {
   bool isInputIncomplete() const { return IsInputIncomplete; }
 
   void checkForInputIncomplete() {
-    IsInputIncomplete = IsInputIncomplete || Tok.is(tok::eof);
+    IsInputIncomplete = IsInputIncomplete ||
+      // Check whether parser reached EOF but the real EOF, not the end of a
+      // string interpolation segment.
+      (Tok.is(tok::eof) && Tok.getText() != ")");
   }
 
   /// \brief This is the current token being considered by the parser.

test/IDE/test-input-complete/Inputs/invalid_interpolation1.swift

@@ -0,0 +1 @@
+foo("\()")

test/IDE/test-input-complete/test_input.swift

@@ -30,6 +30,7 @@
 // RUN: %swift-ide-test -test-input-complete -source-filename %S/Inputs/type_incomplete2.swift | %FileCheck %s -check-prefix=INCOMPLETE
 // RUN: %swift-ide-test -test-input-complete -source-filename %S/Inputs/type_incomplete3.swift | %FileCheck %s -check-prefix=INCOMPLETE
 // RUN: %swift-ide-test -test-input-complete -source-filename %S/Inputs/type_incomplete4.swift | %FileCheck %s -check-prefix=INCOMPLETE
+// RUN: %swift-ide-test -test-input-complete -source-filename %S/Inputs/invalid_interpolation1.swift | %FileCheck %s -check-prefix=COMPLETE
 
 // INCOMPLETE: IS_INCOMPLETE
 // COMPLETE: IS_COMPLETE