Enable -Wconversion (#1359)

Summary:
X-link: https://github.com/facebook/react-native/pull/39291

Pull Request resolved: https://github.com/facebook/yoga/pull/1359

This enables clang warnings around potentially unsafe conversions, such as those with mismatched signedness, or ones which may lead to truncation.

This should catch issues in local development which create errors for MSVC (e.g. Dash), who's default `/W3` includes warnings akin to `-Wshorten-64-to-32`.

This full set of warnings here is a tad spammy, but probably more useful than not.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D48954777

fbshipit-source-id: 1ccc07b99d09d1c2d428158149698ffd04025605

cmake/project-defaults.cmake

@@ -35,6 +35,7 @@ add_compile_options(
     -Wall
     -Wextra
     -Werror
+    -Wconversion
     # Disable RTTI
     $<$<COMPILE_LANGUAGE:CXX>:-fno-rtti>
     # Use -O2 (prioritize speed)