[Coverage] Load records immediately

The current implementation keeps buffers generated for each object file
until it completes loading of all files. This approach requires a lot of memory
if there are a lot of huge object files. Thus, make it to load coverage records
immediately rather than waiting for other binaries to be loaded.

This reduces memory usage of llvm-cov from >128GB to 5GB when
loading Chromium binaries in Windows.

Additional testing: check-profile, check-llvm

Differential Revision: https://reviews.llvm.org/D99110