MC: Unknown assembler directives are now hard errors.

Previously, an unsupported/unknown assembler directive issued a warning.
That's generally unsafe, and inconsistent with the behaviour of pretty
much every system assembler. Now that the MC assemblers are mature
enough to be the default on multiple targets, it's reasonable to
issue errors for these.

For target or platform directives that need to stay warnings, we
should add explicit handlers for them in, e.g., ELFAsmParser.cpp,
DarwinAsmParser.cpp, et. al., and issue the warning there.

rdar://9246275

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155926 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/MC/AsmParser/macros-parsing.s b/test/MC/AsmParser/macros-parsing.s
index 65f6454..75aaac0 100644
--- a/test/MC/AsmParser/macros-parsing.s
+++ b/test/MC/AsmParser/macros-parsing.s
@@ -5,7 +5,7 @@
 .endmacro
 
 .macros_off
-// CHECK-ERRORS: 9:1: warning: ignoring directive for now
+// CHECK-ERRORS: 9:1: error: unknown directive
 .test0
 .macros_on