The last non-partial-specified target can collect and check source files.
Case A:
llvm_add_library(foo PARTIAL_SOURCES_INTENDED ...)
llvm_add_library(bar PARTIAL_SOURCES_INTENDED ...)
llvm_add_library(baz ...)
baz checks source files in foo, bar, and baz.
Case B:
llvm_add_library(hoge PARTIAL_SOURCES_INTENDED ...)
llvm_add_library(fuga PARTIAL_SOURCES_INTENDED ...)
llvm_add_library(piyo PARTIAL_SOURCES_INTENDED ...)
llvm_check_source_file_list() # Explicitly checks sources in hoge, fuga, and piyo.
Differential Revision: https://reviews.llvm.org/D142256