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.
