[MC] - Teach llvm-mc to handle comdats whose names are numbers.

Currently llvm-mc ignores COMDATs whose names are numbers,
for example following code:

.section .foo,"G",@progbits,123,comdat

would produce no COMDATs at all.

Patch fixes the issue. 

Differential revision: https://reviews.llvm.org/D41552

llvm-svn: 321419