llvm-bcanalyzer prints out the stream type of the file it is
analyzing. If the file begins with the LLVM IR magic number, it reports
a stream type of "LLVM IR". However, any other bitstream format is
reported as "unknown".
Add some checks for two other common bitstream formats: Clang AST
files, which begin with 'CPCH', and Clang serialized diagnostics, which
begin with 'DIAG'.
Test Plan: check-llvm
Half-serious: How many implementations of this check do we have now (including clang)? It might be nice to we could add a checkMagic(stdd::array<char, 4>) function to BitStreamReader.