Display PC instead of <unknown> for stack trace in vscode

It isn't useful for users to see "<unknown>" as a stack trace when lldb fails to symbolicate a stack frame. I've replaced "<unknown>" with the value of the program counter instead.

Test Plan:

To test this, I opened a target that lldb fails to symbolicate in
VSCode, and observed in the CALL STACK section that instead of being
shown as "<unknown>", those stack frames are represented by their
program counters.

I added a new test case, `TestVSCode_stackTraceMissingFunctionName` that
exercises this feature.

I also ran `lldb-dotest -p TestVSCode` and saw that the tests passed.

Differential Revision: https://reviews.llvm.org/D156732