Avoid umbrella header crash error in remote compilation when no defau…

…lt umbrella header generated (#897)

### What changed:
1. When generate_default_umbrella_header is set to False, still add
statements around `extern double {module_name}VersionNumber;` to
umbrella header so that clang can resolve the situation when module_name
is the same for two different modules (where their path is diff).
2. When generate_default_umbrella_header is set to False, use `extern`
instead of `FOUNDATION_EXPORT` since `FOUNDATION_EXPORT` is `extern` and
trying to define it in `else` clause will cause `warning: ambiguous
expansion of macro 'FOUNDATION_EXPORT'` error.

### Why this change:
[The original PR](#348) that
introduced the flag `generate_default_umbrella_header` essentially wants
to not import UIKit/Foundation to restrict on what to import by default,
which is a good thing. However, it kinda overkill by also not including
the statements around version number and version string. Which
apparently affects how clang resolves module mapping (from a module name
to a path) and in our set up, leading to error similar to:
```
path/to/alice.modulemap:2:21: error: umbrella for module 'bob' already covers this directory

   umbrella header "foo-umbrella.h"
                    ^
```
For reference, this is originated from `diag::err_mmap_umbrella_clash`
error in https://github.com/llvm/llvm-project

For our case, the umbrella header name for both alice and bob modules
are `foo`, which is totally fine since their path is different. But for
some reason when trying to compile the code in a remote execution
service such as buildfarm, above error shows up. I cannot reproduce it
with `local` or `sandboxed` mode.

Now looking back, the theory is that when there is clash on umbrealla
header name, it will use the module's version string/number to figure
out which is which? I don't really have any other theory for now. I
cannot find such reference to `VersionNumber`/`VersionString` that is
meaningful. Nor can I setup a test case here since it only break remote
execution.

This does not revert the original PR in anyway, I still get to
implicitly import Foundation/UIKit with this change.

### Tests
No change on testing since this flag was not tested anywhere for now. I
can add this flag to some random testing targets in this PR though but
tests so far are still all passing with this turned off.

rules/library.bzl

@@ -142,8 +142,9 @@ def _write_umbrella_header(
         module_name = name
 
     content = ""
-
+    extern_keyword = "extern"
     if generate_default_umbrella_header:
+        extern_keyword = "FOUNDATION_EXPORT"
         content += """\
 #ifdef __OBJC__
 #    import <Foundation/Foundation.h>
@@ -165,13 +166,13 @@ def _write_umbrella_header(
     for header in public_headers:
         content += "#import \"{header}\"\n".format(header = paths.basename(header))
 
-    if generate_default_umbrella_header:
-        content += """
-FOUNDATION_EXPORT double {module_name}VersionNumber;
-FOUNDATION_EXPORT const unsigned char {module_name}VersionString[];
+    content += """
+{extern_keyword} double {module_name}VersionNumber;
+{extern_keyword} const unsigned char {module_name}VersionString[];
 """.format(
-            module_name = module_name,
-        )
+        extern_keyword = extern_keyword,
+        module_name = module_name,
+    )
 
     write_file(
         name = basename + "~",