Tokenized text node may have leading whitespace skipped from their
source-span. But the source-span is used to compute where there are
interpolated blocks, resulting in placeholder nodes whose source-spans
are offset by the amount of skipped characters.
This fix uses the `fullStart` location of text source-spans for computing
the source-span of placeholders, so that they are accurate.
Fixes#39195
PR Close#39486