Give AsmParser an option to control whether it finalizes

Give AsmParser an option to control whether it finalizes
the stream.  New demo:

$ clang asm.c -S -o - -emit-llvm | llc -filetype=obj -o t.o
$ otool -tv t.o
t.o:
(__TEXT,__text) section
_foo:
0000000000000000	subq	$0x08,%rsp
0000000000000004	movl	%edi,(%rsp)
0000000000000007	movl	%edi,%eax
0000000000000009	incl	%eax
000000000000000b	movl	%eax,(%rsp)
000000000000000e	movl	%eax,0x04(%rsp)
0000000000000012	addq	$0x08,%rsp
0000000000000016	ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100492 91177308-0d34-0410-b5e6-96231b3b80d8