[Win] Correct DebugSuffix builds under MSBuild

[Win] Correct DebugSuffix builds under MSBuild
https://bugs.webkit.org/show_bug.cgi?id=139733
<rdar://problem/19276880>

Reviewed by Simon Fraser.

../JavaScriptCore:

* JavaScriptCore.vcxproj/JavaScriptCore.proj: Make sure to use the
'_debug' suffix when building the DebugSuffix target.

../WebKit:

* WebKit.vcxproj/WebKit.proj: Make sure to use the '_debug' suffix
when building the DebugSuffix target.

../WTF:

* WTF.vcxproj/WTF.proj: Make sure to use the '_debug' suffix
when building the DebugSuffix target.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@177448 268f45cc-cd09-0410-ab3c-d52691b4dbfc