[Tooling] Correct the "-std" compile command option.

[hokein]

Summary

"-std c++11" is not valid in compiler, we have to use "-std=c++11".

Test in vscode with this patch, code completion for header works as expected.

[sammccall]

Thanks!

cfe/trunk/lib/Tooling/InterpolatingCompilationDatabase.cpp

if (!TypeCertain) {
                        ? types::lookupHeaderTypeForSourceType(Type)
                        : Type;
       Result.CommandLine.push_back("-x");
       Result.CommandLine.push_back(types::getTypeName(TargetType));
     }
     // --std flag may only be transferred if the language is the same.
     // We may consider "translating" these, e.g. c++11 -> c11.
     if (Std != LangStandard::lang_unspecified && foldType(TargetType) == Type) {
-      Result.CommandLine.push_back("-std");
       Result.CommandLine.push_back(
-          LangStandard::getLangStandardForKind(Std).getName());
+          "-std=" +
+          std::string(LangStandard::getLangStandardForKind(Std).getName()));
     }
     Result.CommandLine.push_back(Filename);
     return Result;
   }
 
 private:
   // Map the language from the --std flag to that of the -x flag.
   static types::ID toType(InputKind::Language Lang) {

cfe/trunk/unittests/Tooling/CompilationDatabaseTest.cpp

 }
 
 TEST_F(InterpolateTest, Language) {
   add("dir/foo.cpp", "-std=c++17");
   add("dir/baz.cee", "-x c");
 
   // .h is ambiguous, so we add explicit language flags
   EXPECT_EQ(getCommand("foo.h"),
-            "clang -D dir/foo.cpp -x c++-header -std c++17");
+            "clang -D dir/foo.cpp -x c++-header -std=c++17");
   // and don't add -x if the inferred language is correct.
-  EXPECT_EQ(getCommand("foo.hpp"), "clang -D dir/foo.cpp -std c++17");
+  EXPECT_EQ(getCommand("foo.hpp"), "clang -D dir/foo.cpp -std=c++17");
   // respect -x if it's already there.
   EXPECT_EQ(getCommand("baz.h"), "clang -D dir/baz.cee -x c-header");
   // prefer a worse match with the right language
   EXPECT_EQ(getCommand("foo.c"), "clang -D dir/baz.cee");
   Entries.erase(path(StringRef("dir/baz.cee")));
   // Now we transfer across languages, so drop -std too.
   EXPECT_EQ(getCommand("foo.c"), "clang -D dir/foo.cpp");
 }