Ensure deterministic when printing ARM assembler constant pools

We dump any non-empty assembler constant pools after a successful
parse of an assembly file that uses the ldr pseudo opcode. These
per-section constant pools should be output in a deterministic order
to ensure that we always generate the same output when printing the
output with an AsmStreamer.

This patch changes the map data struture used to associate a section
with its constant pool to a MapVector to ensure deterministic
output. Because this map type does not support deletion, we now
check that the constant pool is not empty before dumping its entries
and clear the entries after emitting them with the streamer.