Update git submodules

* Update external/scudo from branch 'master'
  to c8c39af172f326bc1eb4f5e63a65db74dac2057c
  - [scudo] Make Scudo compile for C++20
    
    In C++20 compound assignment to volatile (here `LocalData[I]++`) is
    deprecated, so `mutex_test.cpp` fails to compile.
    
    Simply changing it to `LocalData[I] = LocalData[I] + 1` fixes it.
    
    Differential Revision: https://reviews.llvm.org/D117359
    
    GitOrigin-RevId: 636a1cfdcdb8af750a33a6dd2c5d6f84a59aa1c3
    Change-Id: Id413c094b1276c3ba2577d8f51cdfd2b48adb09f
    
1 file changed