Cleanup resource management and rename recursive matchers

This CL follows up on a memory leak issue related to SmallVector growth that
escapes the BumpPtrAllocator.
The fix is to properly use ArrayRef and placement new to define away the
issue.

The following renaming is also applied:
1. MLFunctionMatcher -> NestedPattern
2. MLFunctionMatches -> NestedMatch

As a consequence all allocations are now guaranteed to live on the BumpPtrAllocator.

PiperOrigin-RevId: 231047766