Driver: SCS is compatible with every other sanitizer.

[pcc]

Summary

Because SCS relies on system-provided runtime support, we can use it
together with any other sanitizer simply by linking the runtime for
the other sanitizer.

[eugenis]

LGTM

lib/Driver/SanitizerArgs.cpp

std::pair<SanitizerMask, SanitizerMask> IncompatibleGroups[] = {
       std::make_pair(KernelAddress, Address | Leak | Thread | Memory),
       std::make_pair(HWAddress, Address | Thread | Memory | KernelAddress),
       std::make_pair(Efficiency, Address | HWAddress | Leak | Thread | Memory |
                                      KernelAddress),
       std::make_pair(Scudo, Address | HWAddress | Leak | Thread | Memory |
                                 KernelAddress | Efficiency),
       std::make_pair(SafeStack, Address | HWAddress | Leak | Thread | Memory |
                                     KernelAddress | Efficiency),
-      std::make_pair(ShadowCallStack, Address | HWAddress | Leak | Thread |
-                                          Memory | KernelAddress | Efficiency |
-                                          SafeStack),
       std::make_pair(KernelHWAddress, Address | HWAddress | Leak | Thread |
                                           Memory | KernelAddress | Efficiency |
-                                          SafeStack | ShadowCallStack),
+                                          SafeStack),
       std::make_pair(KernelMemory, Address | HWAddress | Leak | Thread |
                                        Memory | KernelAddress | Efficiency |
                                        Scudo | SafeStack)};
   // Enable toolchain specific default sanitizers if not explicitly disabled.
   SanitizerMask Default = TC.getDefaultSanitizers() & ~AllRemove;
 
   // Disable default sanitizers that are incompatible with explicitly requested
   // ones.

test/Driver/sanitizer-ld.c

 // RUN: %clang -fsanitize=shadow-call-stack %s -### -o %t.o 2>&1 \
 // RUN:     -target x86-unknown-linux -fuse-ld=ld \
 // RUN:   | FileCheck --check-prefix=CHECK-SHADOWCALLSTACK-LINUX-X86 %s
 // CHECK-SHADOWCALLSTACK-LINUX-X86: error: unsupported option '-fsanitize=shadow-call-stack' for target 'x86-unknown-linux'
 
 // RUN: %clang -fsanitize=shadow-call-stack %s -### -o %t.o 2>&1 \
 // RUN:     -fsanitize=safe-stack -target x86_64-unknown-linux -fuse-ld=ld \
 // RUN:   | FileCheck --check-prefix=CHECK-SHADOWCALLSTACK-SAFESTACK %s
-// CHECK-SHADOWCALLSTACK-SAFESTACK: error: invalid argument '-fsanitize=shadow-call-stack' not allowed with '-fsanitize=safe-stack'
+// CHECK-SHADOWCALLSTACK-SAFESTACK-NOT: error:
 
 // RUN: %clang -fsanitize=cfi -fsanitize-stats %s -### -o %t.o 2>&1 \
 // RUN:     -target x86_64-unknown-linux -fuse-ld=ld \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:   | FileCheck --check-prefix=CHECK-CFI-STATS-LINUX %s
 // CHECK-CFI-STATS-LINUX: "{{.*}}ld{{(.exe)?}}"
 // CHECK-CFI-STATS-LINUX: "--whole-archive" "{{[^"]*}}libclang_rt.stats_client-x86_64.a" "--no-whole-archive"
 // CHECK-CFI-STATS-LINUX-NOT: "--whole-archive"