[CMake] Suppress -Wpass-failed warning (llvm#160472)

libstdc++15 makes use of an unroll pragma inside std::find_if(). This
produces a warning if clang fails to unroll the loop. As this pragma is
outside of our control, suppress the warning. Missed transforms are not
something we care about in this context.

Related to llvm#157666.

Author: nikic

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -962,6 +962,13 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
   # Enable -Wctad-maybe-unsupported to catch unintended use of CTAD.
   add_flag_if_supported("-Wctad-maybe-unsupported" CTAD_MAYBE_UNSPPORTED_FLAG)
+
+  # Disable -Wno-pass-failed flag, which reports failure to perform
+  # optimizations suggested by pragmas. This warning is not relevant for LLVM
+  # projects and may be injected by pragmas in libstdc++.
+  # FIXME: Reconsider this choice if warnings from STL headers can be reliably
+  # avoided (https://github.com/llvm/llvm-project/issues/157666).
+  add_flag_if_supported("-Wno-pass-failed" NO_PASS_FAILED_FLAG)
 endif (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE AND NOT LLVM_ENABLE_WARNINGS)