[Parse] Check the SourceFile for #if evaluation

Remove the `EvaluateConditionals` flags from the
parser, and instead query the source file.

This commit also changes ParserUnit such that it
doesn't evaluate #if conditions by default, as
none of its clients appear to require it. The
only client that wasn't explicitly disabling #if
evaluation and is processing the resulting AST is
swift-indent, so this commit also adds a test to
ensure it continues to work correctly with #if
decls.

tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp

@@ -289,8 +289,6 @@ swiftparse_client_node_t SynParser::parse(const char *source) {
   ParserUnit PU(SM, SourceFileKind::Main, bufID, langOpts, tyckOpts,
                 "syntax_parse_module", std::move(parseActions),
                 /*SyntaxCache=*/nullptr);
-  // Evaluating pound conditions may lead to unknown syntax.
-  PU.getParser().EvaluateConditionals = false;
   std::unique_ptr<SynParserDiagConsumer> pConsumer;
   if (DiagHandler) {
     pConsumer = std::make_unique<SynParserDiagConsumer>(*this, bufID);