Determine if sym is the 'primary' destructor, that is, the most-aggregate destructor (the one that is defined as __xdtor)
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enter C++, mangling as the keywords.
See Source File
_cppmangle.d
Documentation: https://dlang.org/phobos/dmd_cppmangle.html
Coverage: https://codecov.io/gh/dlang/dmd/src/master/src/dmd/cppmangle.d
References: Follows Itanium C++ ABI 1.86 section 5.1 http://refspecs.linux-foundation.org/cxxabi-1.86.html#mangling
which is where the grammar comments come from.
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Do mangling for C++ linkage.
This is the POSIX side of the implementation. It exports two functions to C++, toCppMangleItanium and cppTypeInfoMangleItanium.