[lldb] Support querying registers via generic names without alt_names

Update GetRegisterInfoByName() methods to support getting registers
by a generic name independently of alt_name entries in the register
context.  This makes it possible to use generic names when interacting
with gdbserver (that does not supply alt_names).  It also makes it
possible to remove some of the duplicated information from register
context declarations and/or use alt_names for another purpose.

Differential Revision: https://reviews.llvm.org/D108554