[CMake] Disable -Wmisleading-indentation when using gcc

[vabridgers]

Summary

gcc doesn't handle -Wmisleading-indentation correctly, leading to
warnings when using gcc. See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81334 for details. This
change disables -Wmisleading-indentation to avoid warnings when using
gcc, and leaves it enabled for other compilers. This can be re-enabled
when gcc behaves as expected.

[MaskRay]

Consider using CHECK_CXX_SOURCE_COMPILES to check the GCC issue.
Also, please add a gcc.gnu.org/PRxxx comment.

[dblaikie]

(Thanks for taking a look at this - saw a bunch of those notes about misleading indentation on a recent gcc build)

llvm/cmake/modules/HandleLLVMOptions.cmake

if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   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)
 
   # Prevent bugs that can happen with llvm's brace style.
+  if (CMAKE_COMPILER_IS_GNUCXX)
+    add_flag_if_supported("-Wno-misleading-indentation" MISLEADING_INDENTATION_FLAG)
+  else()
     add_flag_if_supported("-Wmisleading-indentation" MISLEADING_INDENTATION_FLAG)
+  endif()
 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()
 
 macro(append_common_sanitizer_flags)
   if (NOT MSVC)