[dthin-lto][cs-irpgo] Avoid assertion with unset profile output path

When using -fcs-generate-profile with distributed thin-lto without
setting a profile output path, we hit an assertion constructing
PGOOptions:

https://github.com/llvm/llvm-project/blob/6041c745f32e8fd60ed24e29e7d919d8d1c87ca6/llvm/lib/Support/PGOOptions.cpp#L36

In non distributed thin-lto settings (local thin-lto for example), this
profile output path is automatically set to a default value if
-fcs-generate-profile is passed.

This patch follows suit, using the default output path if one isn't set.

Author: Nuri Amari

clang/lib/CodeGen/BackendUtil.cpp

@@ -1334,7 +1334,10 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
   // Context sensitive profile.
   if (CGOpts.hasProfileCSIRInstr()) {
     Conf.RunCSIRInstr = true;
-    Conf.CSIRProfile = std::move(CGOpts.InstrProfileOutput);
+    Conf.CSIRProfile = CGOpts.InstrProfileOutput.empty()
+                           ? getDefaultProfileGenName()
+                           : std::move(CGOpts.InstrProfileOutput);
+
   } else if (CGOpts.hasProfileCSIRUse()) {
     Conf.RunCSIRInstr = false;
     Conf.CSIRProfile = std::move(CGOpts.ProfileInstrumentUsePath);

clang/test/CodeGen/distributed-thin-lto/cs-irpgo.c

@@ -0,0 +1,8 @@
+// RUN: mkdir -p %t
+// RUN: %clang -target x86_64-apple-darwin -flto=thin %s -fcs-profile-generate -c -o %t/main.bc
+// RUN: llvm-lto2 run --thinlto-distributed-indexes %t/main.bc -r=%t/main.bc,_main,px -o %t/index
+// RUN: %clang -target x86_64-apple-darwin -fcs-profile-generate -fthinlto-index=%t/main.bc.thinlto.bc %t/main.bc -c -o main.o
+
+int main() {
+    return 0;
+}