[InstrumentationRuntime] Refactor the API (Part 1/N) (NFCI)

Adapters for instrumentation runtimes have to do two basic things:

  1) Load a runtime library.
  2) Install breakpoints in that library.

This logic is duplicated in the adapters for asan and tsan. Factor it
out and document bits of it to make it easier to add new adapters.

I tested this with check-lldb, and double-checked
testcases/functionalities/{a,t}san.

Differential Revision: https://reviews.llvm.org/D23043

llvm-svn: 278367