Add fall-back mode for clang tools.
Add fall-back mode for clang tools.

Run without flags if we cannot load a compilation database. This matches
the behavior of clang itself when simply called with a source file.

Based on a patch by Russell Wallace.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@254599 91177308-0d34-0410-b5e6-96231b3b80d8

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