fix order of export annotation relative to extern and other annotations

[andrurogerz]

Overview

Change the insert position of export macro relative to other annotations and the extern keyword. Specifically, if a symbol has any other annotations or is extern, we position the export macro immediately before the symbol's type location. This ensures the export annotation is inserted after extern and any existing attributes.

Validation

• Updated existing unit tests.
• Added new test cases.
• Ran tests on Linux & Windows

[andrurogerz]

Now that this LLVM PR is merged, it is safe to insert the export macro after the extern keyword. This assumes the export macro will always resume to an __attribute__-style visibility attribute on non-Windows targets rather than a C++ style attribute, which must appear before the extern keyword.