[ELF] Name MergeSyntheticSection using an input section instead of the output section

In a link map, the input section name gives more information. See the updated
merge-entsize.s for an example. The output file is unchanged.

Compiler generated input sections with the SHF_MERGE flag have names such as
.rodata.str1.1 and .rodata.cstN, and are not affected by -fdata-sections.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D149466