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Disallow declaring reserved @@variable names (#3162)

Now that variables named @@myvar are supported (see PR #3073), we should also address the issue of being able to declare a user-defined variable with the same name as a predefined global @@ variable, for example @@SPID.
Babelfish has always allowed such declarations even though referencing the declared variable would actually reference the predefined global @@variable, thus producing a wrong result. For example:

1> declare @@spid int = 123
2> select @@spid
3> go
spid
-----------
      20902     <===== WRONG!

This fix does not allow predefined @@ global variables to be used as user-defined variables or parameters, and will raise a syntax error as expected in T-SQL.
A special case is the @@PGERROR variable which was introduced by Babelfish: when declaring a variable with this name, the error message will additionally point out that @@PGERROR is a reserved name in Babelfish.

Issues Resolved
BABEL-5388: Disallow declaring reserved @@variable names

Signed-off-by: Rob Verschoor rcv@amazon.com

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