Use constexpr for array lengths in ExceptionWithCallstack

[gregtatum]

Description

I'm building on macOS arm64 using clang and encountered a bug with a compile error.

Example error:

[ 50%] Building CXX object src/CMakeFiles/marian.dir/3rd_party/ExceptionWithCallStack.cpp.o /Users/greg/dev/marian-dev/src/3rd_party/ExceptionWithCallStack.cpp:205:30: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
  205 |     void* backtraceAddresses[MAX_NUM_FRAMES];
      |                              ^~~~~~~~~~~~~~
/Users/greg/dev/marian-dev/src/3rd_party/ExceptionWithCallStack.cpp:205:30: note: read of non-const variable 'MAX_NUM_FRAMES' is not allowed in a constant expression
/Users/greg/dev/marian-dev/src/3rd_party/ExceptionWithCallStack.cpp:204:18: note: declared here
  204 |     unsigned int MAX_NUM_FRAMES = 1024;
      |                  ^
/Users/greg/dev/marian-dev/src/3rd_party/ExceptionWithCallStack.cpp:239:27: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
  239 |             char funcName[MAX_FUNCNAME_SIZE]; // working buffer
      |                           ^~~~~~~~~~~~~~~~~
/Users/greg/dev/marian-dev/src/3rd_party/ExceptionWithCallStack.cpp:239:27: note: read of non-const variable 'MAX_FUNCNAME_SIZE' is not allowed in a constant expression
/Users/greg/dev/marian-dev/src/3rd_party/ExceptionWithCallStack.cpp:237:26: note: declared here
  237 |             unsigned int MAX_FUNCNAME_SIZE = 4096;
      |                          ^
2 errors generated.

This change makes it so that the values are constexpr and fixes the build error.

List of changes:

• Added constexpr to two lines

How to test

This shouldn't change any behavior, and just needs a build to verify. I have run make test but the full regression tests don't work on my machine.

Checklist

• I have tested the code manually
• I have run regression tests
• I have read and followed CONTRIBUTING.md
• I have updated CHANGELOG.md