Switch clang_createTranslationUnitFromSourceFile to use ASTUnit::LoadFromCommandLine.

Switch clang_createTranslationUnitFromSourceFile to use ASTUnit::LoadFromCommandLine.
 - This is much faster, as it avoids the overhead of dumping an AST file to disk and reloading it.
 - For debugging purposes, there is a clang_setUseExternalASTGeneration hook which can be used to disable this.

On the Sketch Cocoa app, the speedup is pretty nice, especially when using a PCH file while scanning the source:

Wall time to c-index-test all files (no PCH):
Old: 23.4221
New: 12.3884

Wall time to c-index-test all files (with a PCH, and "local" mode):
Old: 10.9233
New:  1.9038

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90385 91177308-0d34-0410-b5e6-96231b3b80d8