LLVMProcessSources.cmake: Improve PARTIAL_SOURCES_INTENDED.

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