Don't recursively look for a symbol in all imports of imported modules.

Don't recursively look for a symbol in all imports of imported modules.

Given two or more modules that import each other's scope, the current symbol
lookup routines would go round in circles looking through each import from
each module, possibly checking the same module twice or more until all possible
paths are marked as "searched".

Given enough modules, this causes an exponential slowdown in time taken to find
symbols that do exist, and infinite recursion when they don't.

gdb/ChangeLog:
	* d-namespace.c (d_lookup_symbol_imports): Avoid recursive lookups from
	cyclic imports.

gdb/testsuite/ChangeLog:
	* gdb.dlang/circular.c: New file.
	* gdb.dlang/circular.exp: New file.