[MLIR] Allow for multiple gpu modules during translation.

[MLIR] Allow for multiple gpu modules during translation.

This change makes the ModuleTranslation threadsafe by locking on the
LLVMContext. Furthermore, we now clone the llvm module into a new
context when compiling to PTX similar to what the OrcJit does.

Differential Revision: https://reviews.llvm.org/D78207