[llgo] add llgo source path to LLVM_GO_PACKAGES

Summary:
To support the use of LLVM_EXTERNAL_LLGO_SOURCE_DIR,
with llgo situated in a location other than tools/llgo,
we add the llgo source directory to LLVM_GO_PACKAGES.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D21635

llvm-svn: 276828
GitOrigin-RevId: 5fdb3b89da109dbb081081873b09164e1e190d0c