[PCH] Recover gracefully if the ASTReader detects that a file is different

from the one stored in the PCH/AST, while trying to load a SLocEntry.

We verify that all files of the PCH did not change before loading it but this is not enough because:

- The AST may have been 1) kept around, 2) to do queries on it.
- We may have 1) verified the PCH and 2) started parsing.

Between 1) and 2) files may change and we are going to have crashes because the rest of clang
cannot deal with the ASTReader failing to read a SLocEntry.

Handle this by recovering gracefully in such a case, by initializing the SLocEntry
with the info from the PCH/AST as well as reporting failure by the ASTReader.

rdar://10888929

llvm-svn: 151004