[COFF] Pack Name in Symbol as is done in ELF

Summary:
This assumes all symbols are <4GB long, so we can store them as a 32-bit
integer. This reorders the fields so the length appears first, packing
with the other bitfield data in the base Symbol object.

This saved 70MB / 3.60% of heap allocations when linking
browser_tests.exe with no PDB. It's not much as a percentage, but worth
doing. I didn't do performance measurements, I don't think it will be
measurable in time.

Reviewers: ruiu, inglorion, amccarth, aganea

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60297

llvm-svn: 358794