[LangRef] Introduce `noundef` attribute for fully defined function params

LLVM currently does not require function parameters or return values
to be fully initialized, and does not care if they are poison. This can
be useful if the frontend ABI makes no such demands, but may prevent
helpful backend transformations in case they do. Specifically, the C
and C++ languages require all scalar function operands to be fully
determined.

Introducing this attribute is of particular use to MemorySanitizer
today, although other transformations may benefit from it as well.
We can modify MemorySanitizer instrumentation to provide modest (17%)
space savings where `frozen` is present.

This commit only adds the attribute to the Language Reference, and
the actual implementation of the attribute will follow in a separate
commit.

Differential Revision: https://reviews.llvm.org/D82316

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