[Ada] processId: Do not modify already encoded IDs

[Ada] processId: Do not modify already encoded IDs

The processID function is supposed to take a symbol name, and process it
in a way that allows us to look that symbol up.  This patch is adding
a guard to make sure that we do not apply any transformation if we detect
that we are given an already-encoded symbol name.  For instance:

    gv___XR_pck__global_variable___XE

This happens in the case where we are trying to print the value of
a renaming. To do this, we simply parse and evaluate the XR symbol
name as an expression. Without this change, the expression parser
transforms gv___XR_pck__global_variable___XE into somethink like
gv___xr_pck__global_variable___xe, which then screws up the rest
of the renaming evaluation.

gdb/ChangeLog:

        * ada-lex.p (processId): Do not modify already encoded IDs.
        Update function documentation.