NatSpec: Implement `@inheritdoc`

test/libsolidity/syntaxTests/natspec/invalid/docstring_inheritdoc.sol

@@ -0,0 +1,7 @@
+contract C {
+    /// @inheritdoc X
+    function f() internal {
+    }
+}
+// ----
+// DocstringParsingError 9397: (17-34): Documentation tag @inheritdoc references inexistent contract "X".

test/libsolidity/syntaxTests/natspec/invalid/docstring_inheritdoc2.sol

@@ -0,0 +1,10 @@
+contract D {
+}
+
+contract C is D {
+    /// @inheritdoc D
+    function f() internal {
+    }
+}
+// ----
+// DocstringParsingError 4682: (38-55): Documentation tag @inheritdoc references contract "D", but the contract does not contain a function that is overridden by this function.

test/libsolidity/syntaxTests/natspec/invalid/docstring_inheritdoc3.sol

@@ -0,0 +1,11 @@
+contract D {
+    struct S { uint a; }
+}
+
+contract C is D {
+    /// @inheritdoc D.S
+    function f() internal {
+    }
+}
+// ----
+// DocstringParsingError 1430: (63-82): Documentation tag @inheritdoc reference "D.S" is not a contract.

test/libsolidity/syntaxTests/natspec/invalid/docstring_inheritdoc_wrong_type.sol

@@ -0,0 +1,8 @@
+contract C {
+    struct S { uint a; }
+    /// @inheritdoc S
+    function f() internal {
+    }
+}
+// ----
+// DocstringParsingError 1430: (42-59): Documentation tag @inheritdoc reference "S" is not a contract.

test/libsolidity/syntaxTests/natspec/invalid/inherit_doc_events.sol

@@ -0,0 +1,17 @@
+contract ERC20 {
+	/// @notice This event is emitted when a transfer occurs.
+	/// @param from The source account.
+	/// @param to The destination account.
+	/// @param amount The amount.
+	/// @dev A test case!
+	event Transfer(address indexed from, address indexed to, uint amount);
+}
+
+contract A is ERC20 {
+	/// @inheritdoc ERC20
+	event Transfer();
+}
+
+
+// ----
+// DocstringParsingError 6546: (305-326): Documentation tag @inheritdoc not valid for events.