The implementation of `#if` configuration evaluation assumed that the
SourceFile instance within the `Parser` contained all of the source ranges
for code parsed within that `Parser` instance. This is not always the case,
so avoid using the SourceFile directly and instead use the source buffer
that directly contains the condition.
Fixes https://github.com/swiftlang/swift/issues/76137. Big thank you
to Alex Hoppen and the stress tester for finding this.
