Remove references to the 4.0 release as a major breaking (NFC)

This is cleaning up comments (mostly in the bitcode handling) about
removing some backward compatibility aspect in the 4.0 release.
Historically, "4.0" was used during the development of the 3.x
versions as "this future major breaking change version". At the time
the major number was used to indicate the compatibility. When we
reached 3.9 we decided to change the numbering, instead of going to
3.10 we went to 4.0 but after changing the meaning of the major
number to not mean anything anymore with respect to bitcode backward
compatibility.

The current policy
(https://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility)
indicates only now:

  The current LLVM version supports loading any bitcode since version 3.0.

Differential Revision: https://reviews.llvm.org/D82514

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