Organize LLVM C API docs into doxygen modules; add docs

This gives a lot of love to the docs for the C API. Like Clang's
documentation, the C API is now organized into a Doxygen "module"
(LLVMC). Each C header file is a child of the main module. Some modules
(like Core) have a hierarchy of there own. The produced documentation is
thus better organized (before everything was in one monolithic list).

This patch also includes a lot of new documentation for APIs in Core.h.
It doesn't document them all, but is better than none. Function docs are
missing @param and @return annotation, but the documentation body now
commonly provides help details (like the expected llvm::Value sub-type
to expect).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153157 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm-c/Transforms/Vectorize.h

@@ -26,9 +26,20 @@
 extern "C" {
 #endif
 
+/**
+ * @defgroup LLVMCTransformsVectorize Vectorization transformations
+ * @ingroup LLVMCTransforms
+ *
+ * @{
+ */
+
 /** See llvm::createBBVectorizePass function. */
 void LLVMAddBBVectorizePass(LLVMPassManagerRef PM);
 
+/**
+ * @}
+ */
+
 #ifdef __cplusplus
 }
 #endif /* defined(__cplusplus) */