Add 'eager-checks' as a module parameter to MSAN.

This creates a way to configure MSAN to for eager checks that will be leveraged
by the introduction of a clang flag (-fsanitize-memory-param-retval).

This is redundant with the existing flag: -mllvm -msan-eager-checks.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D116855