Fix gdb.mi/mi-fullname-deleted.exp with Clang

Fix gdb.mi/mi-fullname-deleted.exp with Clang

gdb.mi/mi-fullname-deleted.exp attempts to generate an executable
with a doubled slash in the source filename recorded in its debug
info.  This attempt fails when compiling using Clang, causing the
substituted filename test to fail.  This commit fixes this issue.
It also expands the comments in the file, to clarify what the test
is attempting to do, and to document where it doesn't succeed.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-fullname-deleted.exp: Fix substituted
	fullname test with Clang.  Also expand comments generally.