This patch introduces the `vmem-to-lds-load-insts` target feature, which
can be used to enable builtins `__builtin_amdgcn_global_load_lds` and
`__builtin_amdgcn_raw_ptr_buffer_load_lds` on platforms which have this
feature.

This feature is only available on gfx9/10.

A limitation of using a common target feature for both builtins is that
we could have made `__builtin_amdgcn_raw_ptr_buffer_load_lds` available
on gfx6,7,8.

Addressing remarks after merge of D159257

* Add comment
* Remove irrelevant CHECKs from test
* Simplify function
* Use llvm::sort before setting target-features as it is done in
CodeGenModeule

Buitlins from AMD's device-libs are compiled without specifying a
target-cpu, which results in builtins without the target-features
attribute set.

Before this patch, when linking this builtins with -mlink-builtin-bitcode
the target-features were not propagated in the incoming builtins.

With this patch, the default target features are propagated
if they are compatible with the target-features in the incoming builtin.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D159206

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