[Expression] Add PersistentExpressionState::GetCompilerTypeFromPersistentDecl

Summary:
PersistentStateExpressions (e.g. ClangPersistentVariables) have the
ability to define types using expressions that persist throughout the
debugging session. GetCompilerTypeFromPersistentDecl is a useful
operation to have if you need to use any of those persistently declared types,
like in CommandObjectMemory.

This decouples clang from CommandObjectMemory and decouples Plugins from
Commands in general.

Differential Revision: https://reviews.llvm.org/D62797

llvm-svn: 363183