Process input files in Standard JSON mode just like in other modes

- This makes `-` for stdin work.
- `--ignore-missing` now works with `--standard-json`, though it's not very useful because there can be at most one input file.
- Separate errors for situations where there are no input files on the command line (this can be detected in the parser) and where they are not present on disk.

test/solc/Common.cpp

@@ -0,0 +1,58 @@
+/*
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+// SPDX-License-Identifier: GPL-3.0
+
+#include <test/solc/Common.h>
+
+#include <solc/CommandLineInterface.h>
+
+#include <sstream>
+
+using namespace std;
+using namespace solidity::frontend;
+
+vector<char const*> test::makeArgv(vector<string> const& _commandLine)
+{
+	size_t argc = _commandLine.size();
+	vector<char const*> argv(_commandLine.size() + 1);
+
+	// C++ standard mandates argv[argc] to be NULL
+	argv[argc] = nullptr;
+
+	for (size_t i = 0; i < argc; ++i)
+		argv[i] = _commandLine[i].c_str();
+
+	return argv;
+}
+
+test::OptionsReaderAndMessages test::parseCommandLineAndReadInputFiles(
+	vector<string> const& _commandLine,
+	string const& _standardInputContent,
+	bool _processInput
+)
+{
+	vector<char const*> argv = makeArgv(_commandLine);
+	stringstream sin(_standardInputContent), sout, serr;
+	CommandLineInterface cli(sin, sout, serr);
+	bool success = cli.parseArguments(static_cast<int>(_commandLine.size()), argv.data());
+	if (success)
+		success = cli.readInputFiles();
+	if (success && _processInput)
+		success = cli.processInput();
+
+	return {success, cli.options(), cli.fileReader(), cli.standardJsonInput(), sout.str(), serr.str()};
+}