Creating a named struct requires only a Context and a name, but looking up a...

Creating a named struct requires only a Context and a name, but looking up a struct by name requires a Module. The method on Module merely accesses the LLVMContextImpl and no data from the module itself, so this patch moves getTypeByName to a static method on StructType that takes a Context and a name.

There's a small number of users of this function, they are all updated.

This updates the C API adding a new method LLVMGetTypeByName2 that takes a context and a name.

Differential Revision: https://reviews.llvm.org/D78793