[LLVMDebugInfoPDB] - Use cantFail() instead of assert().

Currently injected-sources-native.test fails with "Expected<T>
value was in success state.
(Note: Expected<T> values in success mode must still be checked
prior to being destroyed)"
when llvm is compiled with LLVM_ENABLE_ABI_BREAKING_CHECKS in Release.

The problem is that getStringForID returns Expected<StringRef>
and Expected value must always be checked, even if it is in success state.
Checking with assert only helps in Debug and is wrong.

Differential revision: https://reviews.llvm.org/D69251

llvm-svn: 375492