Add an 'inline hint' attribute to represent source

Add an 'inline hint' attribute to represent source
code hints that it would be a good idea to inline
a function ("inline" keyword).  No functional change
yet; FEs do not emit this and inliner does not use it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80063 91177308-0d34-0410-b5e6-96231b3b80d8