detect-clang: fix version numbers for clang < 4.0

This caused the header to be useless for detecting versions of clang
prior to 4.0.

I also changed a couple of the warnings we use to detect the version;
AFAICT the old ones would have worked, but these seem a bit more
likely to work everywhere.

simde-detect-clang.h

@@ -75,14 +75,14 @@
 #    define SIMDE_DETECT_CLANG_VERSION 50000
 #  elif __has_attribute(diagnose_if)
 #    define SIMDE_DETECT_CLANG_VERSION 40000
-#  elif __has_warning("-Wcast-calling-convention")
-#    define SIMDE_DETECT_CLANG_VERSION 30900
-#  elif __has_warning("-WCL4")
-#    define SIMDE_DETECT_CLANG_VERSION 30800
-#  elif __has_warning("-WIndependentClass-attribute")
-#    define SIMDE_DETECT_CLANG_VERSION 30700
+#  elif __has_warning("-Wcomma")
+#    define SIMDE_DETECT_CLANG_VERSION 39000
+#  elif __has_warning("-Wdouble-promotion")
+#    define SIMDE_DETECT_CLANG_VERSION 38000
+#  elif __has_warning("-Wshift-negative-value")
+#    define SIMDE_DETECT_CLANG_VERSION 37000
 #  elif __has_warning("-Wambiguous-ellipsis")
-#    define SIMDE_DETECT_CLANG_VERSION 30600
+#    define SIMDE_DETECT_CLANG_VERSION 36000
 #  else
 #    define SIMDE_DETECT_CLANG_VERSION 1
 #  endif