Properly detect multiple licenses and validate them.

test/libsolidity/syntaxTests/license/license_AND.sol

@@ -0,0 +1,3 @@
+// SPDX-License-Identifier: GPL-3.0 AND GPL-2.0
+contract C {}
+// ----

test/libsolidity/syntaxTests/license/license_OR.sol

@@ -0,0 +1,3 @@
+// SPDX-License-Identifier: GPL-3.0 OR GPL-2.0
+contract C {}
+// ----

test/libsolidity/syntaxTests/license/license_double2.sol

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0
+// SPDX-License-Identifier: MIT
+contract C {}
+// ----
+// ParserError 3716: Multiple SPDX license identifiers found in source file. Use "AND" or "OR" to combine multiple licenses. Please see https://spdx.org for more information.

test/libsolidity/syntaxTests/license/license_double3.sol

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0
+// SPDX-License-Identifier: GPL-3.0
+contract C {}
+// ----
+// ParserError 3716: Multiple SPDX license identifiers found in source file. Use "AND" or "OR" to combine multiple licenses. Please see https://spdx.org for more information.

test/libsolidity/syntaxTests/license/license_double4.sol

@@ -0,0 +1,5 @@
+/* SPDX-License-Identifier: GPL-3.0 OR GPL-2.0
+SPDX-License-Identifier: GPL-3.0 OR GPL-2.0 */
+contract C {}
+// ----
+// ParserError 3716: Multiple SPDX license identifiers found in source file. Use "AND" or "OR" to combine multiple licenses. Please see https://spdx.org for more information.

test/libsolidity/syntaxTests/license/license_double5.sol

@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: GPL-3.0 OR GPL-2.0 */ /* SPDX-License-Identifier: GPL-3.0 OR GPL-2.0 */
+contract C {}
+// ----
+// ParserError 3716: Multiple SPDX license identifiers found in source file. Use "AND" or "OR" to combine multiple licenses. Please see https://spdx.org for more information.

test/libsolidity/syntaxTests/license/license_hidden_unicode.sol

@@ -1,4 +1,4 @@
-// This is parsed as GPL-3.0:
 // SPDX-License-Identifier: GPL-3.0 ⡉⡊⡋⡌⡍⡎⡏⡐⡑⡒
 contract C {}
 // ----
+// ParserError 1114: Invalid SPDX license identifier.

test/libsolidity/syntaxTests/license/license_unicode.sol

@@ -1,4 +1,4 @@
 // SPDX-License-Identifier: ⡉⡊⡋⡌⡍⡎⡏⡐⡑⡒
 contract C {}
 // ----
-// Warning 1878: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
+// ParserError 1114: Invalid SPDX license identifier.