pgsql: Exclude LLVM files from whitespace checks

Exclude LLVM files from whitespace checks

Commit 9044fc1d45a added some files from upstream LLVM.  These files
have different whitespace rules, which make the git whitespace checks
powered by gitattributes fail.  To fix, add those files to the exclude
list.

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/467df7f561dde102029e20e0011dc98987de47ff

Modified Files
--------------
.gitattributes | 3 +++
1 file changed, 3 insertions(+)