clang-format: [JS] Fix line breaks before comments when sorting imports.

[mprobst]

Summary

Previously, clang-format would always insert an additional line break after the
import block if the main body started with a comment, due to loosing track of
the first non-import line.

[djasper]

Looks Good.

cfe/trunk/lib/Format/SortJavaScriptImports.cpp

for (auto Line : AnnotatedLines) {
       Current = Line->First;
       LineEnd = Line->Last;
       skipComments();
       if (Start.isInvalid() || References.empty())
         // After the first file level comment, consider line comments to be part
         // of the import that immediately follows them by using the previously
         // set Start.
         Start = Line->First->Tok.getLocation();
-      if (!Current)
-        continue; // Only comments on this line.
+      if (!Current) {
+        // Only comments on this line. Could be the first non-import line.
+        FirstNonImportLine = Line;
+        continue;
+      }
       JsModuleReference Reference;
       Reference.Range.setBegin(Start);
       if (!parseModuleReference(Keywords, Reference)) {
-        FirstNonImportLine = Line;
+        if (!FirstNonImportLine)
+          FirstNonImportLine = Line; // if no comment before.
         break;
       }
+      FirstNonImportLine = nullptr;
       AnyImportAffected = AnyImportAffected || Line->Affected;
       Reference.Range.setEnd(LineEnd->Tok.getEndLoc());
       DEBUG({
         llvm::dbgs() << "JsModuleReference: {"
                      << "is_export: " << Reference.IsExport
                      << ", cat: " << Reference.Category
                      << ", url: " << Reference.URL
                      << ", prefix: " << Reference.Prefix;

cfe/trunk/unittests/Format/SortImportsTestJS.cpp

TEST_F(SortImportsTestJS, Comments) {
   verifySort("/** @fileoverview This is a great file. */\n"
              "// A very important import follows.\n"
              "import {sym} from 'a';  /* more comments */\n"
              "import {sym} from 'b';  // from //foo:bar\n",
              "/** @fileoverview This is a great file. */\n"
              "import {sym} from 'b';  // from //foo:bar\n"
              "// A very important import follows.\n"
              "import {sym} from 'a';  /* more comments */\n");
+  verifySort("import {sym} from 'a';\n"
+             "import {sym} from 'b';\n"
+             "\n"
+             "/** Comment on variable. */\n"
+             "const x = 1;\n",
+             "import {sym} from 'b';\n"
+             "import {sym} from 'a';\n"
+             "\n"
+             "/** Comment on variable. */\n"
+             "const x = 1;\n");
 }
 
 TEST_F(SortImportsTestJS, SortStar) {
   verifySort("import * as foo from 'a';\n"
              "import {sym} from 'a';\n"
              "import * as bar from 'b';\n",
              "import {sym} from 'a';\n"
              "import * as foo from 'a';\n"