DwarfDebug: Support floating point constants in location lists.

This patch closes a gap in the DWARF backend that caused LLVM to drop
debug info for floating point variables that were constant for part of
their scope. Floating point constants are emitted as one or more
DW_OP_constu joined via DW_OP_piece.

This fixes a regression caught by the LLDB testsuite that I introduced
in r262247 when we stopped blindly expanding the range of singular
DBG_VALUEs to span the entire scope and started to emit location lists
with accurate ranges instead.

Also deletes a now-impossible testcase (debug-loc-empty-entries).

<rdar://problem/25448338>

llvm-svn: 265760