Improve readability of MinGW driver. NFC.
Improve readability of MinGW driver. NFC.

Summary:
In addition to removing a few global variables and functions, I believe
this patch improves code readability a bit in general.

Reviewers: mstorsjo, martell

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37699

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@312940 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld