Fix hash_for undefined reference in hipTestConstant test

Issue: mismatch undefined symbols in different user env
  - Binary expects modified return value std::string&
  - Fails to match libhip_hcc.so: return value is std::string& but doesn't match modified C++ env
Fix: Change return value to char*, create new key std::string in header from char*

include/hip/hcc_detail/hip_runtime_api.h

@@ -2518,7 +2518,7 @@ struct Agent_global {
 
 namespace hip_impl {
 hsa_executable_t executable_for(hipModule_t);
-const std::string& hash_for(hipModule_t);
+const char* hash_for(hipModule_t);
 
 template<typename ForwardIterator>
 std::pair<hipDeviceptr_t, std::size_t> read_global_description(
@@ -2543,7 +2543,8 @@ hipError_t read_agent_global_from_module(hipDeviceptr_t* dptr, size_t* bytes,
     // hipModule_t instance
     static std::unordered_map<
         std::string, std::vector<Agent_global>> agent_globals;
-    auto key = hash_for(hmod);
+    const char* hash = hash_for(hmod);
+    std::string key(hash);
 
     if (agent_globals.count(key) == 0) {
         static std::mutex mtx;

src/hip_module.cpp

@@ -287,8 +287,8 @@ namespace hip_impl {
         return hmod->executable;
     }
 
-    const std::string& hash_for(hipModule_t hmod) {
-        return hmod->hash;
+    const char* hash_for(hipModule_t hmod) {
+        return hmod->hash.c_str();
     }
 
     hsa_agent_t this_agent() {