[Support] json::Value construction from std::vector<T> and std::map<string,T>.

[sammccall]

Summary

Previously this required a conversion to json::Array/json::Object first.

include/llvm/Support/JSON.h

public:
 
   // It would be nice to have Value() be null. But that would make {} null too.
   Value(const Value &M) { copyFrom(M); }
   Value(Value &&M) { moveFrom(std::move(M)); }
   Value(std::initializer_list<Value> Elements);
   Value(json::Array &&Elements) : Type(T_Array) {
     create<json::Array>(std::move(Elements));
   }
+  template <typename Elt>
+  Value(const std::vector<Elt> &C) : Value(json::Array(C)) {}
   Value(json::Object &&Properties) : Type(T_Object) {
     create<json::Object>(std::move(Properties));
   }
+  template <typename Elt>
+  Value(const std::map<std::string, Elt> &C) : Value(json::Object(C)) {}
   // Strings: types with value semantics. Must be valid UTF-8.
   Value(std::string V) : Type(T_String) {
     if (LLVM_UNLIKELY(!isUTF8(V))) {
       assert(false && "Invalid UTF-8 in value used as JSON");
       V = fixUTF8(std::move(V));
     }
     create<std::string>(std::move(V));
   }

unittests/Support/JSONTest.cpp

TEST(JSONTest, Constructors) {
   EXPECT_EQ("[[[3]]]", s({{{3}}}));
   EXPECT_EQ("[[[]]]", s({{{}}}));
   EXPECT_EQ("[[{}]]", s({{Object{}}}));
   EXPECT_EQ(R"({"A":{"B":{}}})", s(Object{{"A", Object{{"B", Object{}}}}}));
   EXPECT_EQ(R"({"A":{"B":{"X":"Y"}}})",
             s(Object{{"A", Object{{"B", Object{{"X", "Y"}}}}}}));
   EXPECT_EQ("null", s(llvm::Optional<double>()));
   EXPECT_EQ("2.5", s(llvm::Optional<double>(2.5)));
+  EXPECT_EQ("[[2.5,null]]", s(std::vector<std::vector<llvm::Optional<double>>>{
+                                 {2.5, llvm::None}}));
 }
 
 TEST(JSONTest, StringOwnership) {
   char X[] = "Hello";
   Value Alias = static_cast<const char *>(X);
   X[1] = 'a';
   EXPECT_EQ(R"("Hallo")", s(Alias));