[Reland] "Do not apply calling conventions to MSVC entry points"

This patch is a second attempt at fixing a link error for MSVC
entry points when calling conventions are specified using a flag.

Calling conventions specified using flags should not be applied to MSVC
entry points. The default calling convention is set in this case. The
default calling convention for MSVC entry points main and wmain is cdecl.
For WinMain, wWinMain and DllMain, the default calling convention is
stdcall on 32 bit Windows.

Explicitly specified calling conventions are applied to MSVC entry points.

For MinGW, the default calling convention for all MSVC entry points is
cdecl.

First attempt: 4cff1b40
Revert of first attempt: bebfc3b9

Differential Revision: https://reviews.llvm.org/D97941