[NFC][CaptureTracking] Move static function isNonEscapingLocalObject to llvm namespace

Function isNonEscapingLocalObject is a static one within BasicAliasAnalysis.cpp.
It wraps around PointerMayBeCaptured of CaptureTracking, checking whether a pointer
is to a function-local object, which never escapes from the function.

Although at the moment, isNonEscapingLocalObject is used only by BasicAliasAnalysis,
its functionality can be used by other pass(es), one of which I will put up for review
very soon. Instead of copying the contents of this static function, I move it to llvm
scope, and place it amongst other functions with similar functionality in CaptureTracking.

The rationale for the location are:
- Pointer escape and pointer being captured are actually two sides of the same coin
- isNonEscapingLocalObject is wrapping around another function in CaptureTracking

Reviewed By: jdoerfert (Johannes Doerfert)

Differential Revision: https://reviews.llvm.org/D89465