[libclang] Introduce options to control the priority for the threads

[libclang] Introduce options to control the priority for the threads
that libclang creates.

-Introduce CXGlobalOptFlags enum for the new options that can be
 set on the CXIndex object.

-CXGlobalOpt_ThreadBackgroundPriorityForIndexing affects:
  clang_indexSourceFile
  clang_indexTranslationUnit
  clang_parseTranslationUnit
  clang_saveTranslationUnit

-CXGlobalOpt_ThreadBackgroundPriorityForEditing affects:
  clang_reparseTranslationUnit
  clang_codeCompleteAt
  clang_annotateTokens

rdar://9075282

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@153562 91177308-0d34-0410-b5e6-96231b3b80d8