Produce AST even when there are parser errors

test/cmdlineTests/recovery_ast_constructor/args

@@ -0,0 +1 @@
+--error-recovery --ast  --hashes

test/cmdlineTests/recovery_ast_constructor/err

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+recovery_ast_constructor/input.sol:5:27: Error: Expected primary expression.
+    balances[tx.origin] = ; // missing RHS.
+                          ^
+recovery_ast_constructor/input.sol:5:27: Warning: Recovered in Statement at ';'.
+    balances[tx.origin] = ; // missing RHS.
+                          ^
+
+Compilation halted after AST generation due to errors.

test/cmdlineTests/recovery_ast_constructor/input.sol

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+pragma solidity >=0.0.0;
+
+contract Error1 {
+  constructor() public {
+    balances[tx.origin] = ; // missing RHS.
+  }
+
+  // Without error recovery we stop due to the above error.
+  // Error recovery however recovers at the above ';'
+  // There should be an AST for the above, albeit with error
+  // nodes.
+
+  // This function parses properly and should give AST info.
+  function five() public view returns(uint) {
+    return 5;
+  }
+}

test/cmdlineTests/recovery_ast_constructor/output

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+Syntax trees:
+
+
+======= recovery_ast_constructor/input.sol =======
+PragmaDirective
+   Source: "pragma solidity >=0.0.0;"
+ContractDefinition "Error1"
+   Source: "contract Error1 {\n  constructor() public {\n    balances[tx.origin] = ; // missing RHS.\n  }\n\n  // Without error recovery we stop due to the above error.\n  // Error recovery however recovers at the above ';'\n  // There should be an AST for the above, albeit with error\n  // nodes.\n\n  // This function parses properly and should give AST info.\n  function five() public view returns(uint) {\n    return 5;\n  }\n}"
+  FunctionDefinition "" - public
+     Source: "constructor() public {\n    balances[tx.origin] = ; // missing RHS.\n  }"
+    ParameterList
+       Source: "()"
+    ParameterList
+       Source: ""
+    Block
+       Source: "{\n    balances[tx.origin] = ; // missing RHS.\n  }"
+  FunctionDefinition "five" - public - const
+     Source: "function five() public view returns(uint) {\n    return 5;\n  }"
+    ParameterList
+       Source: "()"
+    ParameterList
+       Source: "(uint)"
+      VariableDeclaration ""
+         Type: uint256
+         Source: "uint"
+        ElementaryTypeName uint
+           Source: "uint"
+    Block
+       Source: "{\n    return 5;\n  }"
+      Return
+         Source: "return 5"
+        Literal, token: [no token] value: 5
+           Type: int_const 5
+           Source: "5"