You'll have to wait for the next LR release or use the free DNG converter 8.8 to convert to DNG and import the DNG into LR.
Nobody currently knows when the next release is due. It is most likely that there are no more 5.x releases and the next will be LR6.
Links provided in here.
Note that converting to DNG will not provide lens support in LR, which is what the OP wanted. It will only allow access to raw files that are supported by the converter but not by the particular version of LR.
You're actually right. An idea would be to use the Adobe Profile Downloader. What I don't know is if Adobe shares its latest profiles at the space where the downloader gets the profiles from.
To use a new lens profile provided by ACR 8.8 or the DNG Converter 8.8 (if you don’t have Photoshop) prior to a LR 5.8 or LR 6, you can copy the appropriate lens-specific .lcp file to your users profile area.
The Lens Profile Downloader will only download third party profiles but not ones that are supplied as formal updates so if Adobe has actually provided a profile it’s usually best to use theirs.
On Windows, the profiles that come with ACR and the DNGC are located in the manufacturer subfolder under:
You find the appropriate .lcp file and copy it to your user’s profile folder, which on Windows is here:
The default on Windows is to hide the ProgramData and AppData folders in Explorer so you can either change the options to unhide them or just type in the name in the Explorer address area and hit Enter then continue clicking down into the path further.
On a Mac I’m less sure where the ACR and DNGC installers put lens profile files, so I cannot say where to get them from, but if you find them, you can put them this folder that the Lightroom Queen’s indicates:
Macintosh HD / Users / your username / Library / Application Support / Adobe / CameraRaw / CameraProfiles /
I wasn't aware that the download of DNGC includes the lens profiles.
Thank you everyone for your help, and especially to ssprengel for the detail (and of course The Lightroom Queen).
I found the lcp file for the Ai-S lens in Application Support/Adobe/CameraRaw/LensProfiles/1.0/Nikon/ beneath the top-level library (not the user library). This is on OS X - apologies for not pointing this out at the beginning.
I copied the lcp file to the Lightroom directory specified by Victoria Bampton and alluded to by ssprengel. Hey presto, I can now select the lens profile inside Lightroom and correct all my Ai-S shots. Which are many. This makes a big difference to my workflow, so thank you again.
The Ai-S profile filename is actually "NIKON D3 (Nikon NIKKOR 50mm f1.2 AIS) - RAW.lcp" but inside Lightroom or ACR it shows up simply as the lens name without the D3 prefix. Does the "D3" matter? I use a D600, but am not technical enough to dwell on the differences between the sensors. The visual effect of invoking the profile "looks right" in terms of edge distortion and vignetting, but if I should be careful of any issues please point them out.
Lens profiles for smaller sensor cameras cannot be used for images from larger sensor cameras, but at least within a manufacturer, profiles can be used across various camera bodies, so the particular camera body a profile was created with images from doesn't matter except to give an idea of whether a profile is from a crop-frame or full-frame camera.