Define the semantics of registerMemoryBuffer() when insterting duplicates

to let the first module win. This functionality is tested in LLDB.

include/swift/Serialization/SerializedModuleLoader.h

@@ -315,6 +315,9 @@ public:
 class MemoryBufferSerializedModuleLoader : public SerializedModuleLoaderBase {
 
   struct MemoryBufferInfo {
+    MemoryBufferInfo(std::unique_ptr<llvm::MemoryBuffer> &&buffer,
+                     llvm::VersionTuple userVersion)
+        : buffer(std::move(buffer)), userVersion(userVersion) {}
     std::unique_ptr<llvm::MemoryBuffer> buffer;
     llvm::VersionTuple userVersion;
   };
@@ -363,11 +366,16 @@ public:
   /// discovered in the __swift_ast section of a Mach-O file (or the .swift_ast
   /// section of an ELF file) to the search path.
   ///
+  /// If a module is inserted twice, the first one wins, and the return value is
+  /// false.
+  ///
   /// FIXME: make this an actual import *path* once submodules are designed.
-  void registerMemoryBuffer(StringRef importPath,
+  bool registerMemoryBuffer(StringRef importPath,
                             std::unique_ptr<llvm::MemoryBuffer> input,
                             llvm::VersionTuple version) {
-    MemoryBuffers[importPath] = {std::move(input), version};
+    return MemoryBuffers
+        .insert({importPath, MemoryBufferInfo(std::move(input), version)})
+        .second;
   }
 
   void collectVisibleTopLevelModuleNames(