Rails/SquishedSQLHeredocs not safe for autocorrect

[Edderic]

It is currently unsafe for rubocop -a to add .squish on SQL heredocs. SQL code
in Postgresql breaks when those containing new line comments are squished.

Current code for squishing doesn't address ---type comments:

---

Expected behavior

When a SQL string has a comment, .squish is not appropriate:

For example, when we have some code like so:

<<~SQL
  -- some comment that should not be included
  select * from some_table;
SQL

rubocop -a should not add .squish:

<<~SQL
  -- some comment that should not be included
  select * from some_table;
SQL

Actual behavior

rubocop -a adds a .squish for SQL with comments:

<<~SQL.squish
  -- some comment that should not be included
  select * from some_table;
SQL

Squishing of comments starting with -- leads to Postgresql complaining. See article on Postgresql about how comments starting with -- should have a line break after it:

Syntax Using -- symbol
The syntax for creating a SQL comment in PostgreSQL using -- symbol is:

-- comment goes here
In PostgreSQL, a comment started with -- symbol is similar to a comment starting with # symbol. When using the -- symbol, the comment must be at the end of a line in your SQL statement with a line break after it. This method of commenting can only span a single line within your SQL and must be at the end of the line.

Steps to reproduce the problem

Put this in reproduce_squish_issue.rb:

<<~SQL
  -- some comment that should not be included
  select * from some_table;
SQL

Then run rubocop -a --only Rails/SquishedSQLHeredocs reproduce_squish_issue.rb

RuboCop version

$ rubocop -V
0.88.0 (using Parser 2.7.1.4, rubocop-ast 0.3.0, running on ruby 2.6.6 x86_64-darwin19)