[Support] Fix some Wundef warnings

For the _WIN32 macro, it is the definedness that matters rather than
the value.  Most uses of the macro already rely on the definedness.
This commit fixes the few remaining uses that relied on the value.

Differential Revision: https://reviews.llvm.org/D51105


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340520 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/CodeGenCoverage.cpp

@@ -22,7 +22,7 @@
 
 #if LLVM_ON_UNIX
 #include <unistd.h>
-#elif _WIN32
+#elif defined(_WIN32)
 #include <windows.h>
 #endif
 
@@ -93,7 +93,7 @@ bool CodeGenCoverage::emit(StringRef CoveragePrefix,
     std::string Pid =
 #if LLVM_ON_UNIX
         llvm::to_string(::getpid());
-#elif _WIN32
+#elif defined(_WIN32)
         llvm::to_string(::GetCurrentProcessId());
 #else
         "";