[demangler] Fix buffer growth

The output buffer growth algorithm had a few issues:

a) An off-by-one error in the initial size check, which uses
'>='. This error was safe, but could cause us to reallocate when there
was no need.

b) An inconsistency between the initial size check (>=) and the
post-doubling check (>).  The latter was somewhat obscured by the
swapped operands.

c) There would be many reallocs with an initially-small buffer.  Add a
little initialization hysteresis.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119177