[MSVC] Add support for pragma alloc_text

`#pragma alloc_text` is a MSVC pragma that names the code section where functions should be placed. It only
applies to functions with C linkage.

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

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D125011