[MSVC] Add initial support for MSVC pragma optimize

MSVC's pragma optimize turns optimizations on or off based on the list
passed. At the moment, we only support an empty optimization list.

i.e. `#pragma optimize("", on | off)`

From MSVC's docs:

| Parameter | Type of optimization                             |
|-----------|--------------------------------------------------|
| g         | Enable global optimizations. Deprecated          |
| s or t    | Specify short or fast sequences of machine code  |
| y         | Generate frame pointers on the program stack     |

https://docs.microsoft.com/en-us/cpp/preprocessor/optimize?view=msvc-170

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D125723