[RuntimeDyld][Windows] Allocate space for dllimport things.

[al45tair]

We weren't taking account of the space we require in the stubs for things that are dllimported, and as a result we could hit the assertion failure for running out of stub space. Fix that.

rdar://133473673

[al45tair]

@swift-ci Please test

[al45tair]

Explanation: We use RuntimeDyld to load and relocate JITted code; this works by loading individual sections from the JITted object, but in order to do the relocations it needs to allocate space for stub code. On Windows, it also needs to allocate space for extra pointers when dealing with dllimported things. However, because of the way it works, it needs to allocate all the space for the section, the stubs and the extra pointers in one go, so there is code to compute the extra space required. That code was missing any support for the dllimport pointers, which in turn could lead to runtime failures because we didn't have enough space for the required stubs and pointers. (This also affects LLDB on Windows, FWIW.)
Risk: Extremely low. This only affects Windows.
Original PR: #9071
Reviewed by: @compnerd
Resolves: rdar://133473673
Tests: Tested manually with some of the Swift interpreter tests at a prompt.

[al45tair]

@swift-ci Please test

[al45tair]

Windows build failed checkout; it didn't fail because of this PR.

[compnerd]

@swift-ci please test windows platform