[ORC] Convenience support for COFF external calls and `__imp_` (dllimport) symbols.

For calls to external symbols JITLink usually auto-creates a Global Offset Table (GOT) entry and Procedure Linkage Table (PLT) stub. The GOT entry holds the address of the external, and the PLT stub is an indirect branch that uses the address stored in the GOT entry.

My understanding (COFF experts, come at me!) is that in COFF things are more complicated. External references may be annotated with `dllimport` (permitting some compiler optimizations for external references), and stubs would typically be provided by an import library.

Consider the following example:

```c
void foo(void);
__declspec(dllimport) void bar(void);

void ex(void) {
foo();
bar();
}
```
When compiled with `clang --target=x86_64-pc-windows -S -o - ex.c` this generates:
```asm
ex:
...
callq foo
callq *__imp_bar(%rip) ; access via __imp_ allowed, as declared dllimport
```

Assume that `foo` and `bar` are provided by two different DLLs (`foo.dll` and `bar.dll` respectively). Each of these DLLs will have a corresponding import library (`foo.lib` and `bar.lib`) that define an `__imp_` symbol (containing the address of the _real_ definition), and a thunk symbol with the original name that just branches indirectly through the `__imp_` symbol. E.g. `foo.lib` will contain definitions equivalent to:
```asm
__imp_foo:
.quad