[lld-macho][NFC] Remove redundant StringRef construction

It's only used in one branch, so we were unnecessarily calculating the
length of many symbol names.

Tiny speedup when linking chromium_framework on my M1 Mac mini:

x before.txt
+ after.txt

    N           Min           Max        Median           Avg        Stddev
x  10     3.9917109        4.0418     4.0318099     4.0203902   0.021459873
+  10      3.944725      4.053988     3.9708955     3.9825602   0.037257609
Difference at 95.0% confidence
	-0.03783 +/- 0.0285663
	-0.940953% +/- 0.710536%
	(Student's t, pooled s = 0.0304028)

Differential Revision: https://reviews.llvm.org/D130234