Fix buffer overrun in WindowsResourceCOFFWriter::writeSymbolTable()

Summary:
We were using sprintf(..., "$R06X", <some uint32_t>) to create strings
that are expected to be exactly length 8, but this results in longer
strings if the uint32_t is greater than 0xffffff. This change modifies
the behavior as follows:

 - Uses the loop counter instead of the data offset. This gives us
   sequential symbol names, avoiding collisions as much as possible.

 - Masks the value to 0xffffff to avoid generating names longer than 8
   bytes.

 - Uses formatv instead of sprintf.

Fixes PR35581.

Reviewers: ruiu, zturner

Reviewed By: ruiu

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D41270

llvm-svn: 321030