[analyzer] Fix scan-build and exploded-graph-rewriter tests on Windows.

Detect script locations in a more straightforward way: we don't need to
search for them because we know exactly where they are anyway.

Fix a file path escaping issue in exploded-graph-rewriter with Windows
backslashes in the path.

'REQUIRES: shell' remains in scan-build tests for now, so that to
observe the buildbot reaction on removing it in a cleaner experiment.

Patch by Denys Petrov!

Differential Revision: https://reviews.llvm.org/D76768