XFAIL under Clang tests using labels

gdb/testsuite/
	* gdb.base/label.exp: XFAIL label related tests under Clang.
	* gdb.cp/cplabel.exp: Ditto.
	* gdb.linespec/ls-errs.exp: Refactor tests to execute directly
	and XFAIL under Clang those using labels.
Return by value to coax Clang into emitting the full definition of a test type.

gdb/testsuite/
	* gdb.cp/pr10728-x.cc: Return by value instead of pointer to coax
	Clang into emitting the definition of the type.
	* gdb.cp/pr10728-x.h: Ditto.
	* gdb.cp/pr10728-y.cc: Ditto.
Cause clang to emit the definition of a type used only by pointer

gdb/testsuite/
	* gdb.stabs/gdb11479.c (tag_dummy_enum): introduce a variable to cause
	clang to emit the full definition of type required by the test
	* gdb.stabs/gdb11479.exp (do_test): correct a typo in a test message
Ensure unreferenced static symbols aren't omitted by clang (either marking them __attribute__((used)) or making them non-static)

gdb/testsuite/
       * gdb.base/catch-syscall.c: Make unreferenced statics non-static to
       ensure clang would not discard them.
       * gdb.base/gdbvars.c: Ditto.
       * gdb.base/memattr.c: Ditto.
       * gdb.base/whatis.c: Ditto.
       * gdb.python/py-prettyprint.c: Ditto.
       * gdb.trace/actions.c: Ditto.
       * gdb.cp/ptype-cv-cp.cc: Mark unused global const int as used to
       ensure clang would not discard it.