PR26597, FAIL: gdb.dlang/demangle.exp: _D8demangle4testFI5identZv

PR26597, FAIL: gdb.dlang/demangle.exp: _D8demangle4testFI5identZv

gcc commit 387d0773f3 changed the D demangler, with the following
commit log:

    libiberty: Add support for `in' and `in ref' storage classes.

    The storage class `in' is now a first-class citizen with its own mangle
    symbol, of which also permits `in ref'.  Previously, `in' was an alias
    to `const [scope]', which is a type constructor.

    The mangle symbol repurposed for this is `I', which was originally used
    by identifier types.  However, while TypeIdentifier is part of the
    grammar, it must be resolved to some other entity during the semantic
    passes, and so shouldn't appear anywhere in the mangled name.

    Old tests that are now no longer valid have been removed.

This patch makes the same changes to the gdb testsuite as were made to
the upstream gcc libiberty testsuite.

gdb/testsuite/
	PR 26597
	* gdb.dlang/demangle.exp: Update tests as per gcc commit 387d0773f3.