Put H5T_CONV_ab macros in do..while loops (HDFGroup#3432)

Ever since a recent round of macro cleanup, bin/trace and clang-format
have been bickering over what H5Tconv.c should look like and neither
produces readable code.

This change puts the top-level H5T_CONV_ab macros in do..while loops,
adds appropriate semicolons, and adds the missing H5_CLANG_DIAG_ON|OFF
and H5_GCC_CLANG_DIAG_ON|OFF macros to the list of statement macros
clang-format recognizes. H5Tconv.c is now readable and both bin/trace
and clang-format are happy.

.clang-format

@@ -83,6 +83,10 @@ StatementMacros:
   - H5_END_TAG
   - H5_GCC_DIAG_OFF
   - H5_GCC_DIAG_ON
+  - H5_CLANG_DIAG_OFF
+  - H5_CLANG_DIAG_ON
+  - H5_GCC_CLANG_DIAG_OFF
+  - H5_GCC_CLANG_DIAG_ON
   - H5_LEAVE
   - HGOTO_DONE
   - HMPI_DONE_ERROR