PyCall master trips LLVM assertion on precompile

Using the `Homebrew` build of Julia, which has LLVM assertions enabled by default, we get:

```
julia> using PyCall
INFO: Precompiling module PyCall...
Assertion failed: (ctx->gensym_assigned.at(idx)), function emit_expr, file codegen.cpp, line 3155.

signal (6): Abort trap: 6
__pthread_kill at /usr/lib/system/libsystem_kernel.dylib (unknown line)
ERROR: Failed to precompile PyCall to /Users/sabae/.julia/lib/v0.4/PyCall.ji
in error at /usr/local/Cellar/julia/0.4.5/lib/julia/sys.dylib
in compilecache at loading.jl:400
in require at /usr/local/Cellar/julia/0.4.5/lib/julia/sys.dylib
```

Reproduce the failure with the Homebrew build of Julia and `using PyCall`, then inspect the precompilation path around the reported `codegen.cpp` assertion. Confirm the fix by successfully precompiling and loading PyCall without the LLVM assertion.

索引モデルが issue の本文から書いたものです。