layers: VS2013 compatibility

Remove all usages of constexpr to restore compatibility with Visual Studio
2013.  Also make a handful of other minor edits where the VS2013
compiler was reporting errors.

Change-Id: I3c426bea460a4a902ab306b01deb1d76d60f5fa2
diff --git a/layers/parameter_validation_utils.cpp b/layers/parameter_validation_utils.cpp
index 1efb0c8..b299b02 100644
--- a/layers/parameter_validation_utils.cpp
+++ b/layers/parameter_validation_utils.cpp
@@ -1170,11 +1170,11 @@
                                     "is VK_FALSE, pCreateInfos[%d].pMultisampleState must not be NULL. %s",
                                     i, i, validation_error_map[VALIDATION_ERROR_096005de]);
                 } else {
-                    constexpr const VkStructureType valid_next_stypes[] = {
+                    const VkStructureType valid_next_stypes[] = {
                         LvlTypeMap<VkPipelineCoverageModulationStateCreateInfoNV>::kSType,
                         LvlTypeMap<VkPipelineCoverageToColorStateCreateInfoNV>::kSType,
                         LvlTypeMap<VkPipelineSampleLocationsStateCreateInfoEXT>::kSType};
-                    constexpr const char *valid_struct_names =
+                    const char *valid_struct_names =
                         "VkPipelineCoverageModulationStateCreateInfoNV, "
                         "VkPipelineCoverageToColorStateCreateInfoNV, "
                         "VkPipelineSampleLocationsStateCreateInfoEXT";