Object: Shrink the size of irsymtab::Symbol by a word. NFCI.

Instead of storing an UncommonIndex on the Symbol, use a flag bit to store
whether the Symbol has an Uncommon. This shrinks Chromium's .bc files (after
D32061) by about 1%.

Differential Revision: https://reviews.llvm.org/D32070

llvm-svn: 300514