[patch, fortran] PR27698 - Add diagnostic message for invalid names

[Bernhard Fischer <rep dot nop at aon dot at>]

:ADDPATCH fortran:

According to 3.2.1, a name is
letter [alphanumeric-character]...

While we did reject invalid names, this PR wanted to have a diagnostic
message to be printed along rejecting the invalid name.

The attached patch adds this diagnostic message, adds one testcase to
expect the newly added error message and adjusts testcases that trigger
the new error.

Bootstrapped and regtested with check-gfortran on i686-linux-gnu. Ok for
trunk?

PS:
A vaguely related issue in the area of invalid names is whether or not
-fdollar-ok should allow for the '$' as the very first character in a
name ¹). Accepting it would mean that implicit types may need to be
updated for dollar as first char ²)
Attached patch does not address this issue.

¹)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27698#c6
²)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27698#c11

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