Cleanup of the dwarf_expr_context constructor

Cleanup of the dwarf_expr_context constructor

Move the initial values for dwarf_expr_context class data members
to the class declaration in expr.h.

gdb/ChangeLog:

        * dwarf2/expr.c (dwarf_expr_context::dwarf_expr_context):
        Remove initial data members values.
        * dwarf2/expr.h (dwarf_expr_context): Add initial values
        to the class data members.