[Issue 24882] New: COM class is allocated using GC not malloc

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24882

          Issue ID: 24882
           Summary: COM class is allocated using GC not malloc
           Product: D
           Version: D2
          Hardware: All
                OS: Windows
            Status: NEW
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: alphaglosined at gmail.com

When ``_d_newclassT`` was templified, the COM support was regressed.

Previously it would allocate using malloc, now it allocates using the GC.

https://github.com/dlang/dmd/blob/f1adbd18a2fe262ee7474193383ea579f09e794e/druntime/src/core/lifetime.d#L2742

The reason for this regression is the gate for determining if a class is a COM
class. Previously it used the TypeInfo flag.

https://github.com/dlang/dmd/blob/1317ba12e242c1fffa76d1cb5e1a405745f89021/druntime/src/rt/lifetime.d#L92

COM classes have the linkage D, but are actually C++ classes in ABI. Due to
this they cannot be checked by the linkage, and changing this would either
result in a lie, or not be a useful distinction here.

My suggestion is to add a new trait ``isCOMClass``, it needs to read the field
``com`` and return that from the class declaration.

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