add tracing for loads and stores.
The primary goal is to have more options for data-flow-guided fuzzing,
i.e. use data flow insights to perform better mutations or more agressive corpus expansion.
But the feature is general puspose, could be used for other things too.
Pipe the flag though clang and clang driver, same as for the other SanitizerCoverage flags.
While at it, change some plain arrays into std::array.
Tests: clang flags test, LLVM IR test, compiler-rt executable test.
Reviewed By: morehouse
Differential Revision: https://reviews.llvm.org/D113447
Kostya Serebryany