Fix pascal behavior for class fields with testcase

Fix pascal behavior for class fields with testcase

  Problem reported as PR pascal/17815

Part 1/3: Remember the case pattern that allowed finding a field of this.
File gdb/p-exp.y modified

  This is the fix in the pascal parser (p-exp.y),
to avoid the error that GDB does find normal variables
case insensitively, but not fields of this,
inside a class or object method.

Part 2/3: Add "class" option for pascal compiler
File gdb/testsuite/lib/pascal.exp

This part of the patch series is unchanged.
It adds class option to pascal compiler
which adds the required command line option to
accept pascal class types.

Part 3/3:
New file: gdb/testsuite/gdb.pascal/case-insensitive-symbols.exp
New file: gdb/testsuite/gdb.pascal/case-insensitive-symbols.pas

  Here is an updated version of this test, using Pedro's suggestions.
Test to check that PR 17815 is fixed.