[libc++] Mark __{emplace,push}_back_slow_path as noinline (#94379)

These are almost certainly intended to not be inlined. This
significantly reduces code size when `push_back` and `emplace_back` are
used heavily.

Fixes #94360

Author: philnik777

libcxx/include/__vector/vector.h

@@ -1142,6 +1142,24 @@ vector<_Tp, _Allocator>::__emplace_back_slow_path(_Args&&... __args) {
   return this->__end_;
 }
 
+// This makes the compiler inline `__else()` if `__cond` is known to be false. Currently LLVM doesn't do that without
+// the `__builtin_constant_p`, since it considers `__else` unlikely even through it's known to be run.
+// See https://llvm.org/PR154292
+template <class _If, class _Else>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void __if_likely_else(bool __cond, _If __if, _Else __else) {
+  if (__builtin_constant_p(__cond)) {
+    if (__cond)
+      __if();
+    else
+      __else();
+  } else {
+    if (__cond) [[__likely__]]
+      __if();
+    else
+      __else();
+  }
+}
+
 template <class _Tp, class _Allocator>
 template <class... _Args>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 inline
@@ -1152,12 +1170,14 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 inline
 #endif
     vector<_Tp, _Allocator>::emplace_back(_Args&&... __args) {
   pointer __end = this->__end_;
-  if (__end < this->__cap_) {
-    __emplace_back_assume_capacity(std::forward<_Args>(__args)...);
-    ++__end;
-  } else {
-    __end = __emplace_back_slow_path(std::forward<_Args>(__args)...);
-  }
+  std::__if_likely_else(
+      __end < this->__cap_,
+      [&] {
+        __emplace_back_assume_capacity(std::forward<_Args>(__args)...);
+        ++__end;
+      },
+      [&] { __end = __emplace_back_slow_path(std::forward<_Args>(__args)...); });
+
   this->__end_ = __end;
 #if _LIBCPP_STD_VER >= 17
   return *(__end - 1);