[clang][dataflow] Fix 2 bugs in `MemberExpr` interpretation.

There were two (small) bugs causing crashes in the analysis.  This patch fixes both of them.

1. An enum value was accessed as a class member. Now, the engine gracefully
ignores such member expressions.

2. Field access in `MemberExpr` of struct/class-typed global variables. Analysis
didn't interpret fields of global vars, because the vars were initialized before
the fields were added to the "allowlist". Now, the allowlist is set _before_
init of globals.

Differential Revision: https://reviews.llvm.org/D141384