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

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.

Reviewed By: kyulee

Differential Revision: https://reviews.llvm.org/D115979

Author: ellishg

clang/lib/CodeGen/BackendUtil.cpp

@@ -102,7 +102,7 @@ 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 {

compiler-rt/lib/profile/InstrProfilingMerge.c

@@ -95,8 +95,13 @@ static uintptr_t signextIfWin64(void *V) {
 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;