[DRAFT] [clang] translate if constexpr and constant expressions by their compile-time value

[VladimirMakaev]

The frontend translated if constexpr as a run-time if, and constant expressions (case labels,
enumerators, consteval calls) as run-time code. When a condition reads untranslated library
code, Pulse explores the branch that clang discarded.

#include <limits>

template <typename T>
int* if_signed(int* p) {
  if constexpr (std::numeric_limits<T>::is_signed) {
    return p;
  } else {
    return nullptr;
  }
}

void deref_unsigned_bad(int* p) {
  *if_signed<unsigned>(p) = 1; // Infer missed: null dereference
}

The clang plugin now exports IfStmt::isConstexpr() and the integer value of a ConstantExpr,
which the frontend translates. It drops the discarded branch rather than pruning it, so checkers
that are not path-sensitive ignore it too (RacerD no longer reports lock consistency violations
for locks in disabled branches), and locals read only there are not dead stores. Enumerator
initializers that call functions no longer leave a dangling identifier. Code using magic_enum is
analyzed 3x to 30x faster when its headers are inside --project-root.

Test plan

New tests in cpp/{frontend,pulse}-17, cpp/{frontend,pulse,liveness}-20 and the plugin's
constant_expr.cpp. Enumerators become literals in c/frontend/enumeration;
c/bufferoverrun/global.c loses a false positive and gains CONDITION_ALWAYS_TRUE/FALSE
expectations (disabled by default).