[RuntimeDyld][COFF] Build stubs for COFF dllimport symbols.

Summary:
Enables JIT-linking by RuntimeDyld of COFF objects that contain references to
dllimport symbols. This is done by recognizing symbols that start with the
reserved "__imp_" prefix and building a pointer entry to the target symbol in
the stubs area of the section. References to the "__imp_" symbol are updated to
point to this pointer.

Work in progress: The generic code is in place, but only RuntimeDyldCOFFX86_64
and RuntimeDyldCOFFI386 have been updated to look for and update references to
dllimport symbols.

Reviewers: compnerd

Subscribers: hiraditya, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75884