Set the default for -bbsections-cold-text-prefix to .text.split.

[snehasish]

Summary

After using this for a while, we find that it is generally useful to
have it set to .text.split. by default, removing the need for an
additional -mllvm option.

Please ignore in favour of revision: https://reviews.llvm.org/D88997

llvm/lib/CodeGen/BasicBlockSections.cpp

 using llvm::SmallSet;
 using llvm::SmallVector;
 using llvm::StringMap;
 using llvm::StringRef;
 using namespace llvm;
 
 // Placing the cold clusters in a separate section mitigates against poor
 // profiles and allows optimizations such as hugepage mapping to be applied at a
-// section granularity. Where necessary, users should set this to ".text.split."
-// which is recognized by lld via the `-z keep-text-section-prefix` flag.
+// section granularity. Defaults to ".text.split." which is recognized by lld
+// via the `-z keep-text-section-prefix` flag.
 cl::opt<std::string> llvm::BBSectionsColdTextPrefix(
     "bbsections-cold-text-prefix",
     cl::desc("The text prefix to use for cold basic block clusters"),
-    cl::init(".text.unlikely."), cl::Hidden);
+    cl::init(".text.split."), cl::Hidden);
 
 namespace {
 
 // This struct represents the cluster information for a machine basic block.
 struct BBClusterInfo {
   // MachineBasicBlock ID.
   unsigned MBBNumber;
   // Cluster ID this basic block belongs to.