Build the libtorch CI variants with C++20 (#1476)

[aalhossary]

Description

Fixes #1476. As @Zhang-pchao found there, the torch 2.14 headers require C++20, so under PLUMED's default C++17 configure cannot enable libtorch/libmetatomic and the seven pytorch/ and metatomic/ regtests are skipped. That turns -coverage-mpi- and -doc-mpi- red through PLUMED_ALL_TESTS=yes.

1. ci: add --enable-cxx=20 in the CI step that enables libmetatomic/libtorch.
2. asmjit: C++20 does not accept a template-id as a constructor name, and GCC 11/12 reject ASMJIT_NONCOPYABLE(StringBuilderTmp<N>) and two similar uses. Using the injected class name works in every standard. Three lines.
3. PathMSDBase: cherry-pick of PathMSDBase: drop four omp simd pragmas that cannot vectorize #1437 (already on master). Under C++20 the Intel compiler rejects an omp simd pragma over a loop that copies std::vectors; PathMSDBase: drop four omp simd pragmas that cannot vectorize #1437 removed that pragma together with three others that could not vectorize.

I checked locally that these are the only C++20-specific errors: I syntax-checked every source file with clang + libstdc++ 12 + OpenMP under C++17 and under C++20, and did a full GCC 11 build under C++20.

Target release

I would like my code to appear in release v2.10

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

Tested first on my fork, run 38074095509. The whole run is green, including -coverage-mpi- and -doc-mpi- (the two jobs #1476 turned red) as well as -intel- and -nvhpc-mpi-. On -mpi-: 784 tests performed, 0 not applicable, 0 errors, with all seven pytorch/ and metatomic/ regtests run.