[Clang] Allow bitcode linking when the input is LLVM-IR

Clang provides the `-mlink-bitcode-file` and `-mlink-builtin-bitcode`
options to insert LLVM-IR into the current TU. These are usefuly
primarily for including LLVM-IR files that require special handling to
be correct and cannot be linked normally, such as GPU vendor libraries
like `libdevice.10.bc`. Currently these options can only be used if the
source input goes through the AST consumer path. This patch makes the
changes necessary to also support this when the input is LLVM-IR. This
will allow the following operation:

```
clang in.bc -Xclang -mlink-builtin-bitcode -Xclang libdevice.10.bc
```

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D152391