Tweak the MCExternalSymbolizer to not use the SymbolLookUp() call back

to not guess at a symbol name in some cases.

The problem is that in object files assembled starting at address 0, when
trying to symbolicate something that starts like this:

% cat x.s
_t1:
	vpshufd	$0x0, %xmm1, %xmm0

the symbolic disassembly can end up like this:

% otool -tV x.o 
x.o:
(__TEXT,__text) section
_t1:
0000000000000000	vpshufd	$_t1, %xmm1, %xmm0

Which is in this case produced incorrect symbolication.

But it is useful in some cases to use the SymbolLookUp() call back
to guess at some immediate values.  For example one like this
that does not have an external relocation entry:

% cat y.s
_t1:
	movl	$_d1, %eax
.data
_d1:	.long	0

% clang -c -arch i386 y.s

% otool -tV y.o 
y.o:
(__TEXT,__text) section
_t1:
0000000000000000	movl	$_d1, %eax

% otool -rv y.o 
y.o:
Relocation information (__TEXT,__text) 1 entries
address  pcrel length extern type    scattered symbolnum/value
00000001 False long   False  VANILLA False     2 (__DATA,__data)

So the change is based on it is not likely that an immediate Value
coming from an instruction field of a width of 1 byte, other than branches
and items with relocation, are not likely symbol addresses.

With the change the first case above simply becomes:

% otool -tV x.o 
x.o:
(__TEXT,__text) section
_t1:
0000000000000000	vpshufd	$0x0, %xmm1, %xmm0

and the second case continues to work as expected.

rdar://14863405

llvm-svn: 199698