Fix type of values representing optimized out static members

Fix type of values representing optimized out static members

As reported here:

  https://sourceware.org/ml/gdb/2018-02/msg00019.html

the type of values representing static members that are optimized out is
wrong.  It currently assigns the type of the containing class rather
than the type of the field.  This patch fixes that.

I found a place in m-static.exp already dealing with optimized out
static members, so I just added some gdb_test there.

gdb/ChangeLog:

	* value.c (value_static_field): Assign field type instead of
	containing type when returning an optimized out value.

gdb/testsuite/ChangeLog:

	* gdb.cp/m-static.exp: Check type of optimized out static
	member.