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

[Fangrui Song via cfe-commits]

================
@@ -0,0 +1,16 @@
+// RUN: %clang %s -S -emit-llvm -fprofile-generate -fprofile-continuous -o - | FileCheck %s --check-prefix=IRPGO
----------------
MaskRay wrote:

Without a fixed target triple I believe this command will fail on certain platforms that do not support PGO/coverage.

We typically add `%clang` tests to test/Driver and `%clang_cc1` tests to test/CodeGen, for better layering separation

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