Skip VLA structure field tests when compiling with clang

Skip VLA structure field tests when compiling with clang

Clang fails to compile gdb.base/vla-datatypes.c with the following
error: fields must have a constant size: 'variable length array in
structure' extension will never be supported.  This commit factors
the affected tests out into a new testcase, vla-struct-fields.{exp,c},
which is skipped when the testcase is compiled using clang,

gdb/testsuite/ChangeLog:

	* gdb.base/vla-datatypes.c (vla_factory): Factor out sections
	defining and using VLA structure fields into...
	* gdb.base/vla-struct-fields.c: New file.
	* gdb.base/vla-datatypes.exp: Factor out VLA structure field
	tests into...
	* gdb.base/vla-struct-fields.exp: New file.