[LangRef] Clarify Varargs forwarding for musttail calls.
[LangRef] Clarify Varargs forwarding for musttail calls.

This clarification was suggested by @efriedma in D41335, which uses this
behavior to inline musttail calls with varargs.

Reviewers: hfinkel, efriedma, rnk

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D41861


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@322786 91177308-0d34-0410-b5e6-96231b3b80d8

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