cmd/compile: index line number tables by source file to improve sparsity

cmd/compile: index line number tables by source file to improve sparsity

This reduces allocations and also resolves some
lurking inliner/inlinee line-number match problems.
However, it does add about 1.5% to compile time.

This fixes compiler OOMs seen compiling some large protobuf-
derived inputs.  For compiling the compiler itself,

compilebench -pkg cmd/compile/internal/ssa -memprofile withcl.prof

the numberlines-related memory consumption is reduced from 129MB
to 29MB (about a 5% overall reduction in allocation).

Additionally modified after going over changes with Austin
to remove unused code (nobody called size()) and correct
the cache-clearing code.

I've attempted to speed this up by not using maps, and have
not succeeded.  I'd rather get correct code in now, speed it
up later if I can.

Updates #27739.
Fixes #29279.

Change-Id: I098005de4e45196a5f5b10c0886a49f88e9f8fd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/154617
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>