[lldb] Don't construct the demangled strings while indexing the symbol table

The symbol table needs to demangle all symbol names when building its
index. However, this doesn't require the full mangled name: we only need
the base name and the function declaration context. Currently, we always
construct the demangled string during indexing and cache it in the
string pool as a way to speed up future lookups.

Constructing the demangled string is by far the most expensive step of
the demangling process, because the output string can be exponentially
larger than the input and unless you're dumping the symbol table, many
of those demangled names will not be needed again.

This patch avoids constructing the full demangled string when we can
partially demangle. This speeds up indexing and reduces memory usage.

I gathered some numbers by attaching to Slack:

Before
------

  Memory usage: 280MB
  Benchmark 1: ./bin/lldb -n Slack -o quit
    Time (mean ± σ):      4.829 s ±  0.518 s    [User: 4.012 s, System: 0.208 s]
    Range (min … max):    4.624 s …  6.294 s    10 runs

After
-----

  Memory usage: 189MB
  Benchmark 1: ./bin/lldb -n Slack -o quit
    Time (mean ± σ):      4.182 s ±  0.025 s    [User: 3.536 s, System: 0.192 s]
    Range (min … max):    4.152 s …  4.233 s    10 runs

Differential revision: https://reviews.llvm.org/D118814