C++ domain: support numeric user-defined literals (UDLs) in expressions

[rowan-stein]

C++ User Defined Literals not supported

The code as below

namespace units::si {

inline constexpr auto planck_constant = 6.62607015e-34q_J * 1q_s;

}

causes the following error:

WARNING: Invalid definition: Expected end of definition. [error at 58]
  constexpr auto units::si::planck_constant = 6.62607015e-34q_J * 1q_s
  ----------------------------------------------------------^

According to https://github.com/sphinx-doc/sphinx/blob/3.x/sphinx/domains/cpp.py#L4770 Sphinx seems to not have features for UDLs. Could you please add those?

Observed failure and reproduction steps

• Minimal repro in a .rst file:

  .. cpp:namespace:: units::si
  
  .. cpp:var:: inline constexpr auto planck_constant = 6.62607015e-34q_J * 1q_s

• Build with warnings as errors:

  sphinx-build -b html . _build -W

• Expected current behavior: warning shown above. This is due to the C++ domain parser accepting the floating literal 6.62607015e-34 but not the adjacent UDL suffix q_J, leaving trailing text and triggering "Expected end of definition".

Proposed resolution (summary)

• Extend the C++ domain literal parsing to accept an optional user-defined literal suffix (identifier: [A-Za-z_][A-Za-z0-9_]*) immediately following numeric literals, alongside existing builtin suffixes (u/U/l/L/f/F). Builtin suffixes and UDLs should not be combined on the same literal.
• Add tests covering positive and negative cases for numeric UDLs.

This issue tracks adding numeric UDL support so that the example above parses without warnings.