Parser: avoid skipping inactive code if we are building syntax trees.

If syntax trees are requested, we shouldn't skip inactive code. Notice the
inactive code won't be skipped in SwiftSyntax because we always set
PerformConditionEvaluation false for the in-process parser.

This is mostly needed for testing purposes where we add -verify-syntax-tree
to regular compiler invocations.

rdar://50837165

Author: nkcsgexi

lib/Parse/ParseIfConfig.cpp

@@ -638,6 +638,11 @@ ParserResult<IfConfigDecl> Parser::parseIfConfig(
     SmallVector<ASTNode, 16> Elements;
     if (isActive || !isVersionCondition) {
       parseElements(Elements, isActive);
+    } else if (SyntaxContext->isEnabled()) {
+      // We shouldn't skip code if we are building syntax tree.
+      // The parser will keep running and we just discard the AST part.
+      SmallVector<ASTNode, 16> dropedElements;
+      parseElements(dropedElements, false);
     } else {
       DiagnosticTransaction DT(Diags);
       skipUntilConditionalBlockClose();

test/Syntax/diagnostics_verify.swift

@@ -19,3 +19,7 @@ class { // expected-error {{unknown declaration syntax exists in the source}}
   // expected-error@-4 {{top-level statement cannot begin with a closure expression}}
 
 }
+
+#if swift(<1)
+print("Wat")
+#endif