[ClangImporter] Deduplicate clang buffers representing the same file

Clang can end up with multiple file IDs representing the same file, but
in different contexts. For example, file in the current module always
have file IDs >0, while files in imported modules always have file IDs
<-1. As we end up compiling multiple modules, the same file can be seen
both as the "current" module, and a transitive import. We don't want
multiple Swift buffer IDs for the same file, as it breaks things like
-verify that compare buffer IDs.

rdar://162661286

Author: hnrklssn

lib/ClangImporter/ClangSourceBufferImporter.cpp

@@ -52,7 +52,14 @@ SourceLoc ClangSourceBufferImporter::resolveSourceLocation(
                                        buffer.getBufferIdentifier(),
                                        /*RequiresNullTerminator=*/true)
     };
-    mirrorID = swiftSourceManager.addNewSourceBuffer(std::move(mirrorBuffer));
+    // The same underlying file can exist as multiple clang file IDs. E.g. as
+    // part of the its own module, and then later loaded as an import in another
+    // module.
+    auto IDOpt = swiftSourceManager.getIDForBufferIdentifier(buffer.getBufferIdentifier());
+    if (IDOpt.has_value())
+      mirrorID = IDOpt.value();
+    else
+      mirrorID = swiftSourceManager.addNewSourceBuffer(std::move(mirrorBuffer));
     mirroredBuffers[buffer.getBufferStart()] = mirrorID;
   }
   loc = swiftSourceManager.getLocForOffset(mirrorID, decomposedLoc.second);