[clang] [PGO] Add a clang option -fprofile-continuous that enables PGO continuous mode (PR #124353)

[Fangrui Song via cfe-commits]

================
@@ -785,6 +786,34 @@ static void addPGOAndCoverageFlags(const ToolChain &TC, Compilation &C,
       D.Diag(diag::err_drv_unsupported_option_argument)
           << A->getSpelling() << Val;
   }
+  if (const auto *A = Args.getLastArg(options::OPT_fprofile_continuous)) {
+    if (!PGOGenerateArg && !CSPGOGenerateArg && !ProfileGenerateArg)
+      D.Diag(clang::diag::err_drv_argument_only_allowed_with)
+          << A->getSpelling()
+          << "-fprofile-generate, -fprofile-instr-generate, or "
+             "-fcs-profile-generate";
+    else {
+      CmdArgs.push_back("-fprofile-continuous");
+      // Platforms that require a bias variable:
+      if (T.isOSFuchsia() || T.isOSBinFormatELF() || T.isOSAIX()) {
+        CmdArgs.push_back("-mllvm");
----------------
MaskRay wrote:

I vaguely remember that continuous mode could be used without -runtime-counter-relocation on ELF. I just checked again and we disallow that mode. OK, I think it's fine to use

```
if (T.isOSBinFormatELF() || T.isOSAIX())
```

to match `runtime_reloc = bool(config.host_os in ["AIX", "Linux"])` in compiler-rt/test/profile/lit.cfg.py

https://github.com/llvm/llvm-project/pull/124353