[clang][dataflow] Move NoopAnalysis from unittests to include

This patch moves `Analysis/FlowSensitive/NoopAnalysis.h` from `clang/unittests/` to `clang/include/clang/`, so that we can use it for doing context-sensitive analysis.

Reviewed By: ymandel, gribozavr2, sgatev

Differential Revision: https://reviews.llvm.org/D130304