Merging r327322:

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r327322 | arphaman | 2018-03-12 12:36:29 -0700 (Mon, 12 Mar 2018) | 7 lines

[Tooling] Clear the PreambleSrcLocCache when preamble is discarded during reparsing

This ensures that diagnostics are not remapped to incorrect preamble locations after
the second reparse with a remapped header file occurs.

rdar://37502480

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llvm-svn: 332113