[cmake] Disable the -Wmisleading-indentation warning with GCC

[mstorsjo]

Summary

We do keep using the flag with Clang, which should keep catching
such issues in buildbots.

With GCC, the warning can cause lots of loud notes about the
warning logic getting disabled in large sources.

llvm/cmake/modules/HandleLLVMOptions.cmake

if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
   if (NOT C_WCOMMENT_ALLOWS_LINE_WRAP)
     append("-Wno-comment" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
 
   # Enable -Wstring-conversion to catch misuse of string literals.
   add_flag_if_supported("-Wstring-conversion" STRING_CONVERSION_FLAG)
 
+  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # Disable the misleading indentation warning with GCC; GCC can
+    # produce noisy notes about this getting disabled in large files.
+    # See e.g. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89549
+    check_cxx_compiler_flag("-Wmisleading-indentation" CXX_SUPPORTS_MISLEADING_INDENTATION_FLAG)
+    append_if(CXX_SUPPORTS_MISLEADING_INDENTATION_FLAG "-Wno-misleading-indentation" CMAKE_CXX_FLAGS)
+  else()
     # Prevent bugs that can happen with llvm's brace style.
     add_flag_if_supported("-Wmisleading-indentation" MISLEADING_INDENTATION_FLAG)
+  endif()
 
   # Enable -Wctad-maybe-unsupported to catch unintended use of CTAD.
   add_flag_if_supported("-Wctad-maybe-unsupported" CTAD_MAYBE_UNSPPORTED_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)
   append("-w" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 endif()