[cmake] Add SOURCE_DIR argument to llvm_check_source_file_list
[cmake] Add SOURCE_DIR argument to llvm_check_source_file_list

The motivation is to be able to check sources outside the current
directory. See D31363 for example usage.

Differential Revision: https://reviews.llvm.org/D37859

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@313648 91177308-0d34-0410-b5e6-96231b3b80d8

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