[Frontend] Verify that the bitstream is not empty before reading

the serialised diagnostics

Clang should avoid calling report_fatal_error when the file with the serialised
diagnostics is empty. This commit changes Clang's serialised diagnostic reader,
now it reports an appropriate error instead of crashing.

rdar://31939877

Differential Revision: https://reviews.llvm.org/D35069

llvm-svn: 307384