[CodeGen] Handle recursion in LLVMIRGeneration Timer.

This can happen when emitting a local decl, which triggers
loading a decl imported from an AST file, which we then
hand to the AST consumer. Timer is not allowed to recurse
so an assertion fire. Keep a reference counter to avoid this
problem. LGTM'd by Richard Smith on IRC.

Differential Revision:  https://reviews.llvm.org/D20748

llvm-svn: 276242