basic: fix FreeBSD build failure around usage of assert()

LLVM's `CASReference.h` uses `assert()` without first including `<cassert>`,
which poses a problem here when `CASReference.h` is included first thing in a
submodule.

In FreeBSD's <assert.h>, `assert()` expands to a call to a function that is
declared within header guards.  That is, only the first include of `<assert.h>`
gets the function declaration.

This is module-unfriendly, since it means that if multiple submodules of a
module both include `<assert.h>`, only one of them ends up with a usable
`assert()` macro.  If you haven't (transitively) included the right submodule,
and you haven't included `<assert.h>` yourself, then you get a compile error
that looks like the following:

```
llvm-project/llvm/include/llvm/CAS/CASReference.h:75:5: error: declaration of '__assert' must be imported from module 'LLVM_Utils.Support.MathExtras' before it is required
    assert(InternalRef != getDenseMapEmptyRef() && "Reserved for DenseMapInfo");

/usr/include/assert.h:77:6: note: declaration here is not visible
void __assert(const char *, const char *, int, const char *) __dead2;

swift/include/swift/AST/DeclContext.h:31:10: error: could not build module 'BasicBridging'
#include "swift/Basic/SourceLoc.h"

swift/SwiftCompilerSources/Sources/Basic/SourceLoc.swift:13:8: error: could not build C module 'ASTBridging'
import ASTBridging
```

Ideally `CASReference.h` would include `<cassert>` itself.  But use of
`assert()` without a prior include of `<cassert>` is common in LLVM.  It's even
encouraged by the LLVM "coding standards", which say:

> Use the “assert” macro to its fullest. [...] The “<cassert>” header file is
> probably already included by the header files you are using, so it doesn’t
> cost anything to use it.

Since the include of `CASReference.h` appears to be a temporary workaround, add
to it by including `<cassert>`, too.

Author: mhjacobson

include/swift/Basic/BasicBridging.h

@@ -15,6 +15,7 @@
 
 // Workaround to avoid a compiler error because `cas::ObjectRef` is not defined
 // when including VirtualFileSystem.h
+#include <cassert>
 #include "llvm/CAS/CASReference.h"
 
 #include "swift/Basic/BridgedSwiftObject.h"