[Instrumentation] Share InstrumentationIRBuilder between TSan and SanCov

Factor our InstrumentationIRBuilder and share it between ThreadSanitizer
and SanitizerCoverage. Simplify its usage at the same time (use function
of passed Instruction or BasicBlock).

This class may be used in other instrumentation passes in future.

NFCI.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D125038