go/analysis/passes/modernize: embedlit invalid fix

One of the text edits that the embedlit modernizer
produces is to delete the key value expression and
opening brace in a redundant field type specifier:

T{U: U{f: v, ...}}
  -----

If the kv expression is on its own line, we need
to delete the whole line to avoid leaving white
space:
T{
  U: U{
   f: v,
  }
}

Previously, we deleted the whole line after checking
that the previous element is on a different line
(prevLine < curLine) and the closing brace of the
inner literal is on a different line than its
opening brace. However, this did not take
into account the location of the inner elements.
If the first inner field assignment starts
on the same line as the kv expression, deleting
the entire line produces
invalid code:

B: B{b: 1
}

We should verify that the first inner element
starts on a subsequent line before we delete
the entire line.

Fixes golang/go#80197

Change-Id: Ia6623c8dc329e46626677c2b8157c58e2ee81d32
Reviewed-on: https://go-review.googlesource.com/c/tools/+/795441
Auto-Submit: Madeline Kalil <mkalil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>