Runtimes: treat linker warnings as errors

We were previously silently emitting incorrect code for the standard
libraries and the warnings were being swallowed by the build system.
This was brought to light by the new build system. The code generation
issues have been addressed. In order to prevent a backslide, we would
ideally use `/WX:4217 /WX:4286`. Unfortunately, `clang-cl` does not
support this, so compromise and use `/WX` which treats all linker
warnings as errors.

Author: compnerd

Runtimes/Core/CMakeLists.txt

@@ -144,6 +144,15 @@ add_compile_options(
   $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
   $<$<COMPILE_LANGUAGE:CXX>:-funwind-tables>)
 
+# LNK4049: symbol 'symbol' defined in 'filename.obj' is imported
+# LNK4286: symbol 'symbol' defined in 'filename_1.obj' is imported by 'filename_2.obj'
+# LNK4217: symbol 'symbol' defined in 'filename_1.obj' is imported by 'filename_2.obj' in function 'function'
+#
+# We cannot selectively filter the linker warnings as we do not use the MSVC
+# frontned and `clang-cl` (and `clang`) currently do not support `/WX:nnnn`. As
+# a compromise, treat all linker warnings as errors.
+add_link_options($<$<PLATFORM_ID:Windows>:LINKER:/WX>)
+
 add_compile_options(
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-library-level api>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-runtime-compatibility-version none>"