[ThinLTO] Allow importing of multiple symbols with same GUID

Summary:
The is the clang side of the fix in D55047, to handle the case where
two different modules have local variables with the same GUID because
they had the same source file name at compilation time. Allow multiple
symbols with the same GUID to be imported, and test that this case works
with the distributed backend path.

Depends on D55047.

Reviewers: evgeny777

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D55048

llvm-svn: 347887