This Quality Week work item is motivated by my findings from a few
months back that Crossgen2 PDB generator had been producing bogus
symbol files for almost half a year due to a trivial bug and no
testing in place was able to catch that. This change adds initial
provisions for semantic PDB validation.

In this initial commit I'm adding a new managed app PdbChecker
that uses the DIA library to read a given PDB and optionally check
it for the presence of given symbols. In parallel I'm making test
infra changes that enable selective PDB validation support in
individual tests including checks for the presence of expected
symbols.

This change by itself introduces rudimentary PR / CI validation of
PDB files produced by Crossgen2. As next step I plan to introduce
additional provisions for running this logic for all tests to be
added to one of the Crossgen2-specific pipelines, and validation
of PDBs produced during compilation of the framework assemblies.

Thanks

Tomas

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