clang-format: [JS] support fields with case/switch/default labels.

Summary:
`case:` and `default:` would normally parse as labels for a `switch` block.
However in TypeScript, they can be used in field declarations, e.g.:

    interface I {
      case: string;
    }

This change special cases parsing them in declaration lines to avoid wrapping
them.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D36148

llvm-svn: 310070
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
index 06f096a..67004f2 100644
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -242,6 +242,12 @@
   verifyFormat("var interface = 2;");
   verifyFormat("interface = 2;");
   verifyFormat("x = interface instanceof y;");
+  verifyFormat("interface Test {\n"
+               "  x: string;\n"
+               "  switch: string;\n"
+               "  case: string;\n"
+               "  default: string;\n"
+               "}\n");
 }
 
 TEST_F(FormatTestJS, ReservedWordsMethods) {