[flang] Establish a single source of target information for semantics

Create a TargetCharacteristics class to centralize the few items of
target specific information that are relevant to semantics.  Use the
new class for all target queries, including derived type component layout
modeling.

Future work will initialize this class with target information
provided or forwarded by the drivers, and use it to fold layout-dependent
intrinsic functions like TRANSFER().

Differential Revision: https://reviews.llvm.org/D129018

Updates: Attempts to work around build issues on Windows.