clang-format: [JS] support `interface` as a free standing identifier.

Summary:
`interface` can be used as a fee standing identifier in JavaScript/TypeScript.
This change uses the heuristic of whether it's followed by another identifier
as an indication.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D19240

llvm-svn: 266789
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
index aa79fa3..d23a55e 100644
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -136,6 +136,9 @@
                "};");
   verifyFormat("var struct = 2;");
   verifyFormat("var union = 2;");
+  verifyFormat("var interface = 2;");
+  verifyFormat("interface = 2;");
+  verifyFormat("x = interface instanceof y;");
 }
 
 TEST_F(FormatTestJS, CppKeywords) {