[llvm] [CGData] Lazy loading support for stable function map (PR #151660)

[Nico Weber via llvm-commits]

nico wrote:

Looks like this doesn't build with libc++: http://45.33.8.238/macm1/112476/step_3.txt Probably needs an `#include <map>` in StableFunctionMap.h for `unordered_map`? Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/151660