The parsing error tests in ELF/redefine-symbols.test are not specific to ELF.
Move them to redefine-symbols.test.
Add COFF/redefine-symbols.test for COFF specific tests.
Also fix the documentation regarding --redefine-syms: the old and new
names are separated by whitespace, not an equals sign.
Reviewed By: mstorsjo
Differential Revision: https://reviews.llvm.org/D70036