[DebugInfo][PDB] Fix a signed/unsigned coversion warning

Fix the following warning when compiling with clang (caused by commit
rL343951):

GlobalsStream.cpp:61:33: warning: comparison of integers of different
signs: 'int' and 'uint32_t'

This also avoids double evaluation of `GlobalsTable.HashBuckets.size()`.

llvm-svn: 343957