layers: Fix MSVS conversion warning.

This fixes the following:

warning C4245: 'argument': conversion from 'int' to 'unsigned int', signed/unsigned mismatch

It also modifies the build to enable this warning by default.

Change-Id: If2e6c8d43811162a9a382883b3d55a148975fc37
diff --git a/layers/shader_validation.cpp b/layers/shader_validation.cpp
index ff3a84b..11fcbdc 100644
--- a/layers/shader_validation.cpp
+++ b/layers/shader_validation.cpp
@@ -584,8 +584,8 @@
             unsigned id = insn.word(2);
             unsigned type = insn.word(1);
 
-            int location = value_or_default(var_locations, id, -1);
-            int builtin = value_or_default(var_builtins, id, -1);
+            int location = value_or_default(var_locations, id, static_cast<unsigned>(-1));
+            int builtin = value_or_default(var_builtins, id, static_cast<unsigned>(-1));
             unsigned component = value_or_default(var_components, id, 0);  // Unspecified is OK, is 0
             bool is_patch = var_patch.find(id) != var_patch.end();
             bool is_relaxed_precision = var_relaxed_precision.find(id) != var_relaxed_precision.end();