[Remarks] Silence gcc warning by catching unhandled values in switches

Without this fix gcc (7.4) complains with
 ../lib/Remarks/RemarkParser.cpp: In function 'std::unique_ptr<llvm::remarks::ParserImpl> formatToParserImpl(llvm::remarks::ParserFormat, llvm::StringRef)':
 ../lib/Remarks/RemarkParser.cpp:29:1: error: control reaches end of non-void function [-Werror=return-type]
  }
  ^
 ../lib/Remarks/RemarkParser.cpp: In function 'std::unique_ptr<llvm::remarks::ParserImpl> formatToParserImpl(llvm::remarks::ParserFormat, llvm::StringRef, const llvm::remarks::ParsedStringTable&)':
 ../lib/Remarks/RemarkParser.cpp:38:1: error: control reaches end of non-void function [-Werror=return-type]
  }
  ^

The Format enum currently only contains the value YAML which is indeed
already handled in the switches, but gcc complains anyway.

Adding a default case with an llvm_unreachable silences gcc.

llvm-svn: 365118