[CI] Windows premerge tests built with clang generate many warnings...

[cmtice]

During the migration of premerge testing on Windows from using MSVC cl to using Clang, we discovered that Clang on WIndows generates MANY, MANY warnings. In fact, running 'ninja check-mlir' with Clang on WIndows generated so many warnings for some files that it overwhelmed the sccache system.

For the moment we have disabled 4 warnings (for premerge testing on WIndows with clang) that were responsible for most of the issues: -Wc++98-compat, -Wc++14-compat, -Wunsafe-buffer-usage, and -Wold-style-cast.

At some point the Windows-with-clang builds should be cleaned up, so that they are warning-free.

[boomanaiden154]

Reenabled these warnings in #166838 after some cleanup.