Enable building Flang for Windows (experimental)

[kaadam]

I tried to collect all of merged Windows related changes to tracking the status of Flang on Windows. The list contains PRs which is not merged yet (still waiting for review). if these changes are merged, Flang can be compiled for Windows target out of the box on mater branch and the build could be enabled in CMakefile.

• win: Fix type mismatch char* vs const char* #1196
• win: Fix upperilm header generation #1195
• Use cmake built-in touch command instead of native one #1189
• win: Fix the build on Windows on ARM #1181
• Pass to flang2 the MSVC libraries to be linked to classic-flang-llvm-project#65
• win: Fix the inconsistent usage of dllexport/dllimport #1156
• Remove Fortran entry symbol formats for Windows #1151
• win: Fix the build on Windows #1145
• libomp and libpgmath are dependencies of flangrti's downstream flang #1128
• Disable checking SYM size on Windows #1148
• win: Remove unused function declarations #1135
• win: Fix the WIN64 macro usage #1134
• win: fix include path check #1127
• win: Avoid using 'unistd' header #1119
• win: include winsock header for timeval structure #1118
• win: Define ssize_t on Windows #1117
• Windows fixes, rebase of #1084 #1116
• Add missing OpenMP function to ompstub #1060
• Fix the build on Windows #1058
• machar: fix assertion failure on Windows #1031
• patch flang2 to work on windows #1020
• cmake: disable object depends for Ninja generator #988
• add asprintf #985
• Add Windows ARM64 port to libpgmath #984
• patch scutil to work on windows #981
• patch flang runtime library #980
• rename direct header to avoid windows conflict #979
• Fix unresolved sprintf, fprintf error #772
• Update CMake files to support windows #464

[isuruf]

We also need to port flang-compiler/flang-driver#75 to https://github.com/flang-compiler/classic-flang-llvm-project in order to make flang work with downstream cmake projects.

[kaadam]

@isuruf Will you add this driver changes to classic-flang llvm projects? Or shall we port ot based on your code?

[kaadam]

I haven't listed here, but there is a build fix PR for Windows on ARM #1181 which is still waiting for a review.
I do a double check, but I think we could enable the build for Windows for now.
Of course a 'how to build Flang for Windows' wiki page will available soon.

[kaadam]

The current top of master Flang build is working on Windows X64. Maybe we have a chance to merge #1184 and enable the build for Windows. One thing is left in this issue which related for Windows on ARM.

What's your thoughts @kiranchandramohan @bryanpkc ?