[clang-format] Don't insert break between JS template string and tag identifier

[mitchellwills]

Summary

While it's allowed to have the template string and tag identifier separated by a line break, currently the clang-format output is not stable when a break is forced. Additionally, disallowing a line break makes it clear that the identifier is actually a tag for a template string.

[mprobst]

LGTM.

[mprobst]

Landed as r356447.

clang/lib/Format/TokenAnnotator.cpp

       return false; // must not break in "module foo { ...}"
     if (Right.is(TT_TemplateString) && Right.closesScope())
       return false;
+    // Don't split tagged template literal so there is a break between the tag
+    // identifier and template string.
+    if (Left.is(tok::identifier) && Right.is(TT_TemplateString)) {
+      return false;
+    }
     if (Left.is(TT_TemplateString) && Left.opensScope())
       return true;
   }

clang/unittests/Format/FormatTestJS.cpp

 TEST_F(FormatTestJS, TaggedTemplateStrings) {
   verifyFormat("var x = html`<ul>`;");
   verifyFormat("yield `hello`;");
+  verifyFormat("var f = {\n"
+               "  param: longTagName`This is a ${\n"
+               "                    'really'} long line`\n"
+               "};",
+               "var f = {param: longTagName`This is a ${'really'} long line`};",
+               getGoogleJSStyleWithColumns(40));
 }
 
 TEST_F(FormatTestJS, CastSyntax) {