Explicitly initialize opaque pointer mode in CodeGenAction

Explicitly call `LLVMContext::setOpaquePointers` in `CodeGenAction`
before loading any IR files. With this we use the mode specified on the
command-line rather than lazily initializing it based on the contents of
the IR.

This helps when using `-fthinlto-index` which may end up mixing files
with typed and opaque pointer types which fails when the first file
happened to use typed pointers since we cannot downgrade IR with opaque
pointer types to typed pointer types.

Differential Revision: https://reviews.llvm.org/D137475