@isdefined doesn't work in -g2: `Unbound GlobalRef not allowed in value position`

[KristofferC]

Run julia with -g2:

julia> f() = @isdefined(b)
ff (generic function with 1 method)

julia> f()
Unbound GlobalRef not allowed in value position
Internal error: encountered unexpected error in runtime:
MethodError(f=typeof(Base.string)(), args=(), world=0x0000000000001051)
jl_method_error_bare at /home/kc/julia/src/gf.c:1767
jl_method_error at /home/kc/julia/src/gf.c:1785
jl_lookup_generic_ at /home/kc/julia/src/gf.c:2355 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2370
error at ./error.jl:42
check_op at ./compiler/ssair/verify.jl:41
verify_ir at ./compiler/ssair/verify.jl:214
verify_ir at ./compiler/ssair/verify.jl:67 [inlined]
run_passes at ./compiler/ssair/driver.jl:140
optimize at ./compiler/optimize.jl:186
typeinf at ./compiler/typeinfer.jl:35
typeinf_ext at ./compiler/typeinfer.jl:631
typeinf_ext_toplevel at ./compiler/typeinfer.jl:664
typeinf_ext_toplevel at ./compiler/typeinfer.jl:660
jfptr_typeinf_ext_toplevel_12872 at /home/kc/julia/usr/lib/julia/sys.so (unknown line)
_jl_invoke at /home/kc/julia/src/gf.c:2173 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2374
jl_apply at /home/kc/julia/src/julia.h:1687 [inlined]
jl_type_infer at /home/kc/julia/src/gf.c:300
jl_generate_fptr at /home/kc/julia/src/jitlayers.cpp:313
jl_compile_method_internal at /home/kc/julia/src/gf.c:1924
jl_compile_method_internal at /home/kc/julia/src/gf.c:1878 [inlined]
_jl_invoke at /home/kc/julia/src/gf.c:2184 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2374
jl_apply at /home/kc/julia/src/julia.h:1687 [inlined]
do_call at /home/kc/julia/src/interpreter.c:115
eval_value at /home/kc/julia/src/interpreter.c:204
eval_stmt_value at /home/kc/julia/src/interpreter.c:155 [inlined]
eval_body at /home/kc/julia/src/interpreter.c:575
jl_interpret_toplevel_thunk at /home/kc/julia/src/interpreter.c:669
top-level scope at REPL[2]:1
jl_toplevel_eval_flex at /home/kc/julia/src/toplevel.c:837
jl_toplevel_eval_flex at /home/kc/julia/src/toplevel.c:785
jl_toplevel_eval_in at /home/kc/julia/src/toplevel.c:880
eval at ./boot.jl:360
_jl_invoke at /home/kc/julia/src/gf.c:2173 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2374
eval_user_input at /home/kc/julia/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:139
repl_backend_loop at /home/kc/julia/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:200
start_repl_backend at /home/kc/julia/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:185
#run_repl#42 at /home/kc/julia/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:316
run_repl at /home/kc/julia/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:304
_jl_invoke at /home/kc/julia/src/gf.c:2173 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2374
#855 at ./client.jl:387
jfptr_YY.855_36739 at /home/kc/julia/usr/lib/julia/sys.so (unknown line)
_jl_invoke at /home/kc/julia/src/gf.c:2173 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2374
jl_apply at /home/kc/julia/src/julia.h:1687 [inlined]
do_apply at /home/kc/julia/src/builtins.c:672
jl_f__apply_latest at /home/kc/julia/src/builtins.c:722
#invokelatest#2 at ./essentials.jl:709 [inlined]
invokelatest at ./essentials.jl:708 [inlined]
run_main_repl at ./client.jl:372
exec_options at ./client.jl:302
_start at ./client.jl:485
jfptr__start_26500 at /home/kc/julia/usr/lib/julia/sys.so (unknown line)
_jl_invoke at /home/kc/julia/src/gf.c:2173 [inlined]
jl_apply_generic at /home/kc/julia/src/gf.c:2374
jl_apply at /home/kc/julia/src/julia.h:1687 [inlined]
true_main at /home/kc/julia/src/jlapi.c:553
repl_entrypoint at /home/kc/julia/src/jlapi.c:672
main at /home/kc/julia/cli/loader_exe.c:46
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
_start at /home/kc/julia/julia (unknown line)
false

Discovered in #37874 where the doctests were made to run with -g2.