[Hashing] get_execution_seed: use a non-vague linkage symbol

Follow-up to #96282.
Since llvm/lib/Target files are compiled with -fvisibility=hidden,
the seed variable in libLLVM.so is hidden in a -DLLVM_BUILD_LLVM_DYLIB=on build.

This would cause `hash_value(std::string()), hash_value(StringRef())` to
fail since the former (might be part of the main executable) and the
latter (llvm/lib/Support/StringRef.cpp in libLLVM.so) use copies in
different components.

Author: MaskRay

llvm/include/llvm/ADT/Hashing.h

@@ -307,16 +307,16 @@ struct hash_state {
 };
 
 /// In LLVM_ENABLE_ABI_BREAKING_CHECKS builds, the seed is non-deterministic
-/// (address of a variable) to prevent having users depend on the particular
-/// hash values. On platforms without ASLR, this is still likely
-/// non-deterministic per build.
+/// per process (address of a function in LLVMSupport) to prevent having users
+/// depend on the particular hash values. On platforms without ASLR, this is
+/// still likely non-deterministic per build.
 inline uint64_t get_execution_seed() {
   // Work around x86-64 negative offset folding for old Clang -fno-pic
   // https://reviews.llvm.org/D93931
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS &&                                         \
     (!defined(__clang__) || __clang_major__ > 11)
-  static const char seed = 0;
-  return static_cast<uint64_t>(reinterpret_cast<uintptr_t>(&seed));
+  return static_cast<uint64_t>(
+      reinterpret_cast<uintptr_t>(&install_fatal_error_handler));
 #else
   return 0xff51afd7ed558ccdULL;
 #endif