Add simple runtime tests for shadowcallstack

Summary:
ShadowCallStack does not yet have a runtime provided by compiler-rt, but
this change includes simple tests that make use of a very minimal
runtime in test/shadowcallstack/minimal_runtime.h

Reviewers: pcc, kcc, delcypher, eugenis, filcab

Reviewed By: pcc

Subscribers: kubamracek, mgorny, delcypher, llvm-commits, #sanitizers, kcc

Differential Revision: https://reviews.llvm.org/D44803

llvm-svn: 329210