docs: Convert GarbageCollection.html to reST

Patch by Alexander Zinenko!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169702 91177308-0d34-0410-b5e6-96231b3b80d8

docs/subsystems.rst

@@ -22,6 +22,7 @@ Subsystem Documentation
    SystemLibrary
    SourceLevelDebugging
    WritingAnLLVMBackend
+   GarbageCollection
 
 .. FIXME: once LangRef is Sphinxified, HowToUseInstrMappings should be put
    under LangRef's toctree instead of this page's toctree.
@@ -49,9 +50,9 @@ Subsystem Documentation
     
    Information on how to write a new alias analysis implementation or how to
    use existing analyses.
-    
-* `Accurate Garbage Collection with LLVM <GarbageCollection.html>`_
-    
+
+* :doc:`GarbageCollection`
+
    The interfaces source-language compilers should use for compiling GC'd
    programs.