Bitcode: Clean up error handling for certain bitcode query functions.

The functions getBitcodeTargetTriple(), isBitcodeContainingObjCCategory(),
getBitcodeProducerString() and hasGlobalValueSummary() now return errors
via their return value rather than via the diagnostic handler.

To make this work, re-implement these functions using non-member functions
so that they can be used without the LLVMContext required by BitcodeReader.

Differential Revision: https://reviews.llvm.org/D26532

llvm-svn: 286623