Several Changes:

* Fix parallel build problem on generated dependency files
* Fix rule confusion between .a and .la libraries so that parallel builds
  don't get confused on who is building which .o and which library it is
  going into.
* Fix dependency inclusion to only include C/C++ dependency files because
  other types of sources won't have dependencies auto generated.
* Change "Source" to "SOURCES" for naming consistency
* Update parallel build rules for new recursive targets
* Implement EXPERIMENTAL_DIRS (failure allowed) feature
* Implement -local version of targets (all-local, clean-local, etc)
* Implement recursive targets in terms of their local counterparts
* Clarify names of some internal variables
* Move documentation to docs/MakefileGuide.html
* Clean up commentary

llvm-svn: 17192