[demangler] More properly save and restore the template parameter state
when parsing an encoding.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b03f1756fb4fd5ac5d606a7e4fd8aea1d9f18541
2 files changed