[CodeGen] Handle recursion in LLVMIRGeneration Timer.
[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


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@276242 91177308-0d34-0410-b5e6-96231b3b80d8

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