remove some @deprecated markers: LLVM APIs aren't deprecated, they are removed when obsolete.
These APIs are still used, and the constant APIs are actually really important.

Removing these makes -Wdocumentation more useful.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184170 91177308-0d34-0410-b5e6-96231b3b80d8