MDEV-37615: Clang based static analysis false postive reports on UNINIT_VAR

Clang upstream bug llvm/llvm-project#173210
shows that a "int x=x" construct will in the intermediate representation
have code to read from x. With this generated sanitizer=undefined and
static analyzers will all see the uninitialized read and write.

Because clang has a stronger implementation of following paths to
uninitialized variables, "#define UNINIT_VAR(x) x" is the which
is what our release binaries use is the ideal path for this compiler.

Clang based compilers with error during compilation if any
uninitialized behaviour is detected at compile time because of
0c80ddb.

Corrects MDEV-36542 - 6fd57f4.

Author: grooverdan

include/my_global.h

@@ -444,11 +444,10 @@ extern "C" int madvise(void *addr, size_t len, int behav);
 /*
    Suppress uninitialized variable warning without generating code.
 */
-#if defined(__GNUC__) && !defined(WITH_UBSAN)
+#if defined(__GNUC__) && !defined(__clang__)
 /*
   GCC specific self-initialization which inhibits the warning.
-  clang and static analysis will complain loudly about this
-  so compile those under WITH_UBSAN.
+  clang and static analysis will complain loudly about this.
 */
 #define UNINIT_VAR(x) x= x
 #elif defined(FORCE_INIT_OF_VARS)