Add syntax and semantic tests for underscore

test/libsolidity/semanticTests/underscore/as_function.sol

@@ -0,0 +1,20 @@
+contract C {
+    function _() public pure returns (uint) {
+        return 88;
+    }
+
+    function g() public pure returns (uint){
+        return _();
+    }
+
+    function h() public pure returns (uint) {
+        _;
+        return 33;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// _() -> 88
+// g() -> 88
+// h() -> 33

test/libsolidity/semanticTests/underscore/in_function.sol

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+contract C {
+    function f() public pure returns (uint) {
+        uint _;
+        return _;
+    }
+
+    function g() public pure returns (uint) {
+        uint _ = 1;
+        return _;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0
+// g() -> 1

test/libsolidity/semanticTests/underscore/in_modifier.sol

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+contract C {
+    modifier m() {
+        _;
+    }
+
+    modifier n() {
+        string memory _ = "failed";
+        _;
+        revert(_);
+    }
+
+    function f() m() public returns (uint) {
+        return 88;
+    }
+
+    function g() n() public returns (uint) {
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// f() -> 88
+// g() -> FAILURE, hex"08c379a0", 0x20, 6, "failed"

test/libsolidity/syntaxTests/underscore/as_function.sol

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+contract C {
+    function _() public pure {
+    }
+
+    function g() public pure {
+        _();
+    }
+
+    function h() public pure {
+        _;
+    }
+}

test/libsolidity/syntaxTests/underscore/in_function.sol

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+contract C {
+    function f() public pure returns (uint) {
+        uint _;
+        return _;
+    }
+
+    function g() public pure returns (uint) {
+        uint _ = 1;
+        return _;
+    }
+
+    function h() public pure {
+        _;
+    }
+}
+// ----
+// DeclarationError 7576: (230-231): Undeclared identifier.

test/libsolidity/syntaxTests/underscore/in_modifier.sol

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+contract C {
+    modifier m() {
+        _;
+    }
+
+    modifier n() {
+        string memory _ = "";
+        _;
+        revert(_);
+    }
+
+    function f() m() public {
+    }
+
+    function g() n() public {
+    }
+}
+// ----