[memprof] Replace the block cache with a hashmap.

The existing implementation uses a cache + eviction based scheme to
record heap profile information. This design was adopted to ensure a
constant memory overhead (due to fixed number of cache entries) along
with incremental write-to-disk for evictions. We find that since the
number to entries to track is O(unique-allocation-contexts) the overhead
of keeping all contexts in memory is not very high. On a clang workload,
the max number of unique allocation contexts was ~35K, median ~11K.
For each context, we (currently) store 64 bytes of data - this amounts
to 5.5MB (max). Given the low overheads for a complex workload, we can
simplify the implementation by using a hashmap without eviction.

Other changes:
* Memory map is dumped at the end rather than startup. The relative
order in the profile dump is unchanged since we no longer have evicted
entries at runtime.
* Added a test to check meminfoblocks are merged.

Differential Revision: https://reviews.llvm.org/D111676

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