Replace lit feature keyword 'non_COFF' with 'uses_COFF'.

[probinson]

Summary

Part of converting negative feature keywords to positive.
Giving Lang a chance to reject if he prefers 'non_COFF', the only uses are in the JIT tests.

llvm/trunk/test/ExecutionEngine/MCJIT/weak-function.ll

 ; RUN: lli -jit-kind=mcjit -extra-module %p/Inputs/weak-function-2.ll %s
-; REQUIRES: not_COFF
+; UNSUPPORTED: uses_COFF
 ;
 ; Check that functions in two different modules agree on the address of weak
 ; function 'baz'
 ; Testing on COFF platforms is disabled as COFF has no representation of 'weak'
 ; linkage.
 
 define weak i32 @baz() {
 entry:

llvm/trunk/test/ExecutionEngine/OrcMCJIT/weak-function.ll

 ; RUN: lli -jit-kind=orc-mcjit -extra-module %p/Inputs/weak-function-2.ll %s
-; REQUIRES: not_COFF
+; UNSUPPORTED: uses_COFF
 ;
 ; Check that functions in two different modules agree on the address of weak
 ; function 'baz'.
 ; Testing on COFF platforms is disabled as COFF has no representation of 'weak'
 ; linkage.
 
 define weak i32 @baz() {
 entry:

llvm/trunk/test/lit.cfg.py

 # Features
 known_arches = ["x86_64", "mips64", "ppc64", "aarch64"]
 if (config.host_ldflags.find("-m32") < 0
     and any(config.llvm_host_triple.startswith(x) for x in known_arches)):
   config.available_features.add("llvm-64-bits")
 
 config.available_features.add("host-byteorder-" + sys.byteorder + "-endian")
 
+if sys.platform in ['win32']:
+    # ExecutionEngine, no weak symbols in COFF.
+    config.available_features.add('uses_COFF')
+else:
     # Others/can-execute.txt
-if sys.platform not in ['win32']:
     config.available_features.add('can-execute')
-    config.available_features.add('not_COFF')
 
 # Loadable module
 # FIXME: This should be supplied by Makefile or autoconf.
 if sys.platform in ['win32', 'cygwin']:
     loadable_module = (config.enable_shared == 1)
 else:
     loadable_module = True