gdb/rust: Handle printing structures containing strings

gdb/rust: Handle printing structures containing strings

When printing a rust structure that contains a string GDB can
currently fail to read the fields that define the string. This is
because GDB mistakenly treats a value that is the parent structure as
though it is the structure that defines the string, and then fails to
find the fields needed to extract a string.

The solution is to create a new value to represent the string field of
the parent value.

gdb/ChangeLog:

	* rust-lang.c (val_print_struct): Handle printing structures
	containing strings.

gdb/testsuite/ChangeLog:

	* gdb.rust/simple.exp: Add new test case.
	* gdb.rust/simple.rs (struct StringAtOffset): New struct.
	(main): Initialise an instance of the new struct.