Guile: add value-const-value

The Guile API doesn't currently have an equivalent to the Python API's
gdb.Value.const_value(). This commit adds a procedure with equivalent
semantics to the Guile API.

gdb/ChangeLog:

	* NEWS (Guile API): Note the addition of the new procedure.
	* guile/scm-value.c (gdbscm_value_const_value): Add
	implementation of value-const-value procedure.
	(value_functions): Add value-const-value procedure.

gdb/doc/ChangeLog:

	* guile.texi (Values From Inferior In Guile): Add documentation
	for value-const-value.

gdb/testsuite/ChangeLog:

	* gdb.guile/scm-value.exp (test_value_in_inferior): Add test for
	value-const-value.