[CSSPGO] Do not import pseudo probe desc in thinLTO

Previously we reliedy on pseudo probe descriptors to look up precomputed GUID during probe emission for inlined probes. Since we are moving to always using unique linkage names, GUID for functions can be computed in place from dwarf names. This eliminates the need of importing pseudo probe descs in thinlto, since those descs should be emitted by the original modules.

This significantly reduces thinlto memory footprint in some extreme case where the number of imported modules for a single module is massive.

Test Plan:

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D105248