Here is a second version of the patch after incorporating the 
suggestions and adding a test case.

I should also explain what the patch is doing:

Traditionally gdb only supported using directives only for anonymous 
name spaces. It did that by adding to the static block (block 
representing compilation unit scope) a list of using directives which 
state that namespace x is visible in scope y.

This patch make it so that every block not just the static block can own 
a list of using directives. That list states that namespace x is visible 
in 'this' block. The patch also initializes this list during the 
exploration of dies.

During the exploration of the die tree, when a new scope is started the 
context corresponding to the parent still unfinished scope is pushed 
into a context stack (i.e. local variables and parameters for the outer 
scope are saved aside until the exploration of the current one is done) 
once the current scope is finished the information is saved in a block 
object and the parent context is restored (poped). This patch add 
initialization of using directives parallel to the pattern described above.

this patch can be seen on the branch origin/archer-swagiaal-using-directive

git branch archer-swagiaal-using-directive 
origin/archer-swagiaal-using-directive