[InstrProf] Don't profile merge by default in lightweight mode

[ellis]

Summary

Profile merging is not supported when using debug info profile
correlation because the data section won't be in the binary at runtime.
Change the default profile name in this mode to default_%p.proflite so
we don't use profile merging.

[kyulee]

For clarification, the lightweight pgo does not support merging raw data from memory (at runtime) due to missing structural data, but it does support merging raw files offline.
Would you update the comment or error message?

[ellis]

Expand warning message

clang/lib/CodeGen/BackendUtil.cpp

 namespace llvm {
 extern cl::opt<bool> DebugInfoCorrelate;
 }
 
 namespace {
 
 // Default filename used for profile generation.
 std::string getDefaultProfileGenName() {
-  return DebugInfoCorrelate ? "default_%m.proflite" : "default_%m.profraw";
+  return DebugInfoCorrelate ? "default_%p.proflite" : "default_%m.profraw";
 }
 
 class EmitAssemblyHelper {
   DiagnosticsEngine &Diags;
   const HeaderSearchOptions &HSOpts;
   const CodeGenOptions &CodeGenOpts;
   const clang::TargetOptions &TargetOpts;
   const LangOptions &LangOpts;

compiler-rt/lib/profile/InstrProfilingMerge.c

 #else
   return (uintptr_t)V;
 #endif
 }
 
 COMPILER_RT_VISIBILITY
 int __llvm_profile_merge_from_buffer(const char *ProfileData,
                                      uint64_t ProfileSize) {
-  if (__llvm_profile_get_version() & VARIANT_MASK_DBG_CORRELATE)
+  if (__llvm_profile_get_version() & VARIANT_MASK_DBG_CORRELATE) {
+    PROF_ERR(
+        "%s\n",
+        "Debug info correlation does not support profile merging at runtime. "
+        "Instead, merge raw profiles using the llvm-profdata tool.");
     return 1;
+  }
 
   __llvm_profile_data *SrcDataStart, *SrcDataEnd, *SrcData, *DstData;
   __llvm_profile_header *Header = (__llvm_profile_header *)ProfileData;
   uint64_t *SrcCountersStart;
   const char *SrcNameStart;
   const char *SrcValueProfDataStart, *SrcValueProfData;
   uintptr_t CountersDelta = Header->CountersDelta;