[LLVM][Maintainers] Update lead maintainer for LLVM

Set myself as the current lead maintainer for LLVM, and Chris
Lattner as the emeritus lead maintainer.

This implements https://discourse.llvm.org/t/rfc-proposing-a-new-lead-maintainer-for-llvm/81290.

Author: nikic

llvm/Maintainers.md

@@ -16,8 +16,8 @@ assistance.
 
 The lead maintainer is responsible for all parts of LLVM not covered by somebody else.
 
-Chris Lattner \
-[email protected] (email), [lattner](https://github.com/lattner) (GitHub), clattner (Discourse)
+Nikita Popov \
+[email protected], [email protected] (email), [nikic](https://github.com/nikic) (GitHub), nikic (Discourse)
 
 ### Transforms and analyses
 
@@ -426,6 +426,9 @@ help with the success of the project!
 
 ### Emeritus lead maintainers
 
+Chris Lattner \
+[email protected] (email), [lattner](https://github.com/lattner) (GitHub), clattner (Discourse)
+
 ### Inactive component maintainers
 
 ### Former maintainers of removed components