[docs] Add GwpAsan to toctree.

Reverts rL369556 in the process, as it's no longer needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369560 91177308-0d34-0410-b5e6-96231b3b80d8

docs/GwpAsan.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 ========
 GWP-ASan
 ========

docs/index.rst

@@ -206,6 +206,7 @@ For developers of applications which use LLVM as a library.
    FuzzingLLVM
    ScudoHardenedAllocator
    OptBisect
+   GwpAsan
 
 :doc:`LLVM Language Reference Manual <LangRef>`
   Defines the LLVM intermediate representation and the assembly form of the
@@ -258,6 +259,9 @@ For developers of applications which use LLVM as a library.
 :doc:`OptBisect`
   A command line option for debugging optimization-induced failures.
 
+:doc:`GwpAsan`
+  A sampled heap memory error detection toolkit designed for production use.
+
 .. _index-subsystem-docs:
 
 Subsystem Documentation