Don't use $ as suffix for symbol names in ThinLTOBitcodeWriter and other places

Using $ breaks demangling of the symbols. For example,

$ c++filt _Z3foov\$123
_Z3foov$123

This causes problems for developers who would like to see nice stack traces
etc., but also for automatic crash tracking systems which try to organize
crashes based on the stack traces.

Instead, use the period as suffix separator, since Itanium demanglers normally
ignore such suffixes:

$ c++filt _Z3foov.123
foo() [clone .123]

This is already done in some places; try to do it everywhere.

Differential revision: https://reviews.llvm.org/D97484