[Object] Fix reading objects created with -fembed-bitcode-marker

Currently, this fails with many tools, e.g.

$ clang -fembed-bitcode-marker -c -o test.o test.c
$ nm test.o
nm: test.o The file was not recognized as a valid object file

-fembed-bitcode-marker creates a LLVM,bitcode section consisting of a single
byte. When reading the object file, IRObjectFile::findBitcodeInObject succeeds,
causing SymbolicFile::createSymbolicFile to try to read the "bitcode" rather
than using the outer Mach-O data - when then fails.

Fix this by making findBitcodeInObject return an error if the section size <= 1.

Patched by: Nicholas Allegra

Differential Revision: https://reviews.llvm.org/D44373

llvm-svn: 356718