misc tests SIGSEGV in ClassLoaderData::is_alive()

The following test failed in the JDK15 CI:

java/foreign/TestLayoutPaths.java

Here's a snippet from the log file:

#section:testng
----------messages:(5/232)----------
command: testng TestLayoutPaths
reason: User specified action: run testng TestLayoutPaths
Mode: othervm [test needs --add-modules]
Additional options from @modules: --add-modules jdk.incubator.foreign
elapsed time (seconds): 7.611
----------configuration:(3/49)----------
Boot Layer
  add modules: jdk.incubator.foreign

----------System.out:(21/1660)----------
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x00007fb473e96c20, pid=31425, tid=31441
#
# JRE version: Java(TM) SE Runtime Environment (15.0+26) (build 15-ea+26-1286)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (15-ea+26-1286, mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x4eec20] ClassLoaderData::is_alive() const+0x0
#
# Core dump will be written. Default location: Core dumps may be processed with "/opt/core.sh %p" (or dumping to /opt/mach5/mesos/work_dir/slaves/805146e6-8fdb-4552-bf9e-385b73cf7129-S259/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/3a50beb0-7178-47a0-8372-e05302aebdc1/runs/5ac78805-6dbf-48d6-9b0e-13295c0f3d24/testoutput/test-support/jtreg_open_test_jdk_tier1_part3/scratch/2/core.31425)
#
# An error report file with more information is saved as:
# /opt/mach5/mesos/work_dir/slaves/805146e6-8fdb-4552-bf9e-385b73cf7129-S259/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/3a50beb0-7178-47a0-8372-e05302aebdc1/runs/5ac78805-6dbf-48d6-9b0e-13295c0f3d24/testoutput/test-support/jtreg_open_test_jdk_tier1_part3/scratch/2/hs_err_pid31425.log
#
# Compiler replay data is saved as:
# /opt/mach5/mesos/work_dir/slaves/805146e6-8fdb-4552-bf9e-385b73cf7129-S259/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/3a50beb0-7178-47a0-8372-e05302aebdc1/runs/5ac78805-6dbf-48d6-9b0e-13295c0f3d24/testoutput/test-support/jtreg_open_test_jdk_tier1_part3/scratch/2/replay_pid31425.log
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#
----------System.err:(0/0)----------
----------rerun:(36/4775)*----------


Here's the crashing thread's stack:

--------------- T H R E A D ---------------

Current thread (0x00007fb46c20a0d0): JavaThread "C1 CompilerThread0" daemon [_thread_in_vm, id=31441, stack(0x00007fb454577000,0x00007fb454678000)]


Current CompileTask:
C1: 125 35 ! 3 java.util.concurrent.ConcurrentHashMap::putVal (432 bytes)

Stack: [0x00007fb454577000,0x00007fb454678000], sp=0x00007fb454675a88, free space=1018k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x4eec20] ClassLoaderData::is_alive() const+0x0
V [libjvm.so+0x57a482] ClassHierarchyWalker::find_witness_anywhere(Klass*, bool, bool)+0x352
V [libjvm.so+0x57bf6b] Dependencies::find_unique_concrete_method(Klass*, Method*)+0x6b
V [libjvm.so+0x4b4d42] ciMethod::find_monomorphic_target(ciInstanceKlass*, ciInstanceKlass*, ciInstanceKlass*, bool)+0x132
V [libjvm.so+0x3ea6a6] GraphBuilder::invoke(Bytecodes::Code)+0x13e6
V [libjvm.so+0x3eaeab] GraphBuilder::iterate_bytecodes_for_block(int)+0x5cb
V [libjvm.so+0x3ed3ed] GraphBuilder::iterate_all_blocks(bool)+0x6d
V [libjvm.so+0x3ef4c6] GraphBuilder::GraphBuilder(Compilation*, IRScope*)+0x316
V [libjvm.so+0x3f4eca] IRScope::IRScope(Compilation*, IRScope*, int, ciMethod*, int, bool)+0x1ca
V [libjvm.so+0x3f4f83] IR::IR(Compilation*, ciMethod*, int)+0xa3
V [libjvm.so+0x3cf453] Compilation::build_hir() [clone .part.0]+0x163
V [libjvm.so+0x3d03ac] Compilation::compile_java_method()+0x3bc
V [libjvm.so+0x3d0518] Compilation::compile_method()+0x108
V [libjvm.so+0x3d098c] Compilation::Compilation(AbstractCompiler*, ciEnv*, ciMethod*, int, BufferBlob*, DirectiveSet*)+0x20c
V [libjvm.so+0x3d14dd] Compiler::compile_method(ciEnv*, ciMethod*, int, DirectiveSet*)+0xad
V [libjvm.so+0x5333cb] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x32b
V [libjvm.so+0x5347a8] CompileBroker::compiler_thread_loop()+0x4e8
V [libjvm.so+0xcd49be] JavaThread::thread_main_inner()+0xde
V [libjvm.so+0xcd98fd] Thread::call_run()+0xfd
V [libjvm.so+0xb30ad7] thread_native_entry(Thread*)+0xe7


siginfo: si_signo: 11 (SIGSEGV), si_code: 1 (SEGV_MAPERR), si_addr: 0x0000000000000024


This is a Tier1 failure so I'm starting it as a P2.
The crashing stack is in compiler code so I'm putting
it in hotspot/compiler for initial triage.