[XCOFF] Use a single symbolic constant for the size of an embeded name. [NFC]

Convert SymbolNameSize and SectionNameSize into just `NameSize`. The length of
a name embeded in a symbol table entry or section header table entry is length 8
for Sections, Symbols and Files. No need to have a distinct constant for each
one. Also removes the Size argument to 'generateStringRef' as the size is
always 'XCOFF::NameSize'.

llvm-svn: 368584