Add .index-build to .gitignore (#77977)

The `.index-build` directory is created by SourceKit-LSP when background indexing is enabled, contains auto-generated transient files similar in purpose to `.build`, and it should be ignored.

Author: MaxDesiatov

.gitignore

@@ -52,6 +52,7 @@ docs/_build
 
 # SwiftPM
 .build
+.index-build
 .swiftpm
 
 #==============================================================================#