Provide default --cs-profile-path= for distributed thin-lto builds

When -fcs-profile-generate is used with a distributed compile without
specifying a profile output path somehow, we fail the following
assertion:

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

Avoid the assertion by choosing a default output path as other LTO mechanisms
(ie. local thin-lto) do.

Author: Nuri Amari

clang/lib/CodeGen/BackendUtil.cpp

@@ -1334,7 +1334,7 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
   // Context sensitive profile.
   if (CGOpts.hasProfileCSIRInstr()) {
     Conf.RunCSIRInstr = true;
-    Conf.CSIRProfile = std::move(CGOpts.InstrProfileOutput);
+    Conf.CSIRProfile = getProfileGenName(CGOpts);
   } 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 -fcs-profile-generate -flto=thin -c -o %t/main.bc %s
+// RUN: llvm-lto2 run --thinlto-distributed-indexes %t/main.bc -o %t/index -r=%t/main.bc,_main,px
+// RUN: %clang -target x86_64-apple-darwin -fcs-profile-generate -c %t/main.o -fthinlto-index=%t/main.bc.thinlto.bc %t/main.bc
+
+int main() {
+    return 0;
+}