[LLDB] Make sure that indexing is done before clearing DIE info

"ClearDIEs()" was being called too soon, before everyone was done using the DIEs.

This fix delays the calls to ::ClearDIEs() until all compile units have been indexed.

1 - Call "::ExtractDIEsIfNeeded()" on all compile units on separate threads. See if each CU has the DIEs parsed and remember this.
2 - Index all compile units on separate threads.
3 - Clear any DIEs in any compile units that didn't have their DIEs parsed after all compile units have been indexed.

Patch by phlav

Differential Revision: http://reviews.llvm.org/D20738

llvm-svn: 271209