Description:
compiler-rt provides the runtime library for ASAN which also contains functions for debuggers. Due to stripping its symbols, this happens though:

$ echo 'int main(int argc, char *argv[]) {}' > main.c
$ clang -fsanitize=address -g main.c
$ gdb -q -batch -ex start -ex 'call __asan_describe_address(argv)' ./a.out
'__asan_describe_address' has unknown return type; cast the call to its declared return type

Linking against libasan.so from GCC or against a local compiler-rt build with symbols gives the expected result:
$ clang -fsanitize=address -g main.c -c
$ clang main.o /usr/lib/libasan.so.5 -pthread -ldl -lrt
$ gdb -q -batch -ex start -ex 'call __asan_describe_address(argv)' ./a.out
Address 0x7fffffffe398 is located in stack of thread T0

Current workaround is to cast the result:
(gdb) call (void)__asan_describe_address(argv)

Additional info:
* compiler-rt 6.0.0-1