Implement some functions in NativeSession.

Summary:
This change implements readFromExe, and calculating VA and RVA, which
are some of the functionalities that will be used for native PDB reading
for llvm symbolizer.

bug: https://bugs.llvm.org/show_bug.cgi?id=41795

Reviewers: hans, amccarth, rnk

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78128