[sanitizers CMake] NFC Refactor the logic for compiling and generating tests

into a function.

Most CMake configuration under compiler-rt/lib/*/tests have
almost-the-same-but-not-quite functions of the form add_X_[unit]tests
for compiling and running the tests.
Much of the logic is duplicated with minor variations across different
sub-folders.
This can harm productivity for multiple reasons:

For newcomers, resulting CMake files are very large, hard to understand,
and hide the intention of the code.
Changes for enabling certain architectures end up being unnecessarily
large, as they get duplicated across multiple folders.
Adding new sub-projects requires more effort than it should, as a
developer has to again copy-n-paste the configuration, and it's not even
clear from which sub-project it should be copy-n-pasted.
With this change the logic of compile-and-generate-a-set-of-tests is
extracted into a function, which hopefully makes writing and reading
CMake much easier.

Differential Revision: https://reviews.llvm.org/D36116

llvm-svn: 310971