Fix duplicate symbol linker error when building Windows tests w/ coverage (#942)

This fixes the fatalError seen on Windows when attempting to build tests
with code coverage enabled. The linker doesn't allow duplicate symbols
by default on Windows, while this is OK on macOS/Linux.

Use the `-lldmingw` linker flag to work around this issue.

Author: plemarquand

src/debugger/buildConfig.ts

@@ -336,6 +336,20 @@ export class TestingDebugConfigurationFactory {
         if (isRelease(this.testKind)) {
             result = [...result, "-c", "release", "-Xswiftc", "-enable-testing"];
         }
+        // `link.exe` doesn't support duplicate weak symbols, and lld-link in an effort to
+        // match link.exe also doesn't support them by default. We can use `-lldmingw` to get
+        // lld-link to allow duplicate weak symbols, but that also changes its library search
+        // path behavior, which could(?) be unintended.
+        //
+        // On the `next` branch (6.1?) in llvm, the duplicate symbol behavior now has its own flag
+        // `-lld-allow-duplicate-weak` (https://github.com/llvm/llvm-project/pull/68077).
+        // Once this is available we should use it if possible, as it will suppress the warnings
+        // seen with `-lldmingw`.
+        //
+        // SEE: rdar://129337999
+        if (process.platform === "win32" && this.testKind === TestKind.coverage) {
+            result = [...result, "-Xlinker", "-lldmingw"];
+        }
         return result;
     }