Bitcode: Remove the remnants of the BitcodeDiagnosticInfo class.

The BitcodeReader no longer produces BitcodeDiagnosticInfo diagnostics.
The only remaining reference was in the gold plugin; the code there has been
dead since we stopped producing InvalidBitcodeSignature error codes in r225562.
While at it remove the InvalidBitcodeSignature error code.

llvm-svn: 286326

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6621,8 +6621,6 @@ class BitcodeErrorCategoryType : public std::error_category {
   std::string message(int IE) const override {
     BitcodeError E = static_cast<BitcodeError>(IE);
     switch (E) {
-    case BitcodeError::InvalidBitcodeSignature:
-      return "Invalid bitcode signature";
     case BitcodeError::CorruptedBitcode:
       return "Corrupted bitcode";
     }