[clang][MSVC] Correct mangling of thread-safe static initialization variables. (#85300)

Static local variables with dynamic initializers depend on implicitly defined
guard variables to synchronize thread-safe initialization.  These guard
variables may have external linkage and therefore require a stable name for
linkage purposes.  The Microsoft ABI assigns these variables a local name of
'$TSS' followed by a discriminator and mangles them as a static local variable
of type 'int'.  Previously, the '$TSS<discriminator>' portion of the name was
not registered as a back reference candidate and this resulted in incorrect
back references for enclosing class and/or namespace scopes that might be
referenced in the signature of the enclosing function.  This change adds the
previously missing back reference registration.  This matches the mangling
performed by MSVC and resolves incompatibilities when inline functions with
static local variables are inlined across DLL boundaries.

This is an ABI change and has the potential to cause backward compatibility
issues with previous Clang releases.

Fixes #83616