system includes are wonky on macos 10.15

[shrouxm]

running the indexer on itself on macos 10.15 produces the following error:

[efritz]

Will update readme to reflect change:

$ clang -print-resource-dir
/Library/Developer/CommandLineTools/usr/lib/clang/11.0.3

$ lsif-clang --extra-arg='-resource-dir=/Library/Developer/CommandLineTools/usr/lib/clang/11.0.3' --executor=all-TUs compile_commands.json > dump.lsif
[1/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/Trace.cpp
[2/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/LSIFSerialization.cpp
[4/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/SymbolID.cpp
[6/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/Logger.cpp
[7/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/SymbolCollector.cpp
[5/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/CodeCompletionStrings.cpp
[11/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/ExpectedTypes.cpp
[8/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/FuzzyMatch.cpp
[12/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/Protocol.cpp
[10/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/Merge.cpp
[13/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/SymbolLocation.cpp
[14/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/indexer/IndexerMain.cpp
[3/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/Index.cpp
[15/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/URI.cpp
[16/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/SourceCode.cpp
[9/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/Headers.cpp
[17/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/IndexAction.cpp
[18/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/Symbol.cpp
[19/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/Ref.cpp
[20/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/SymbolOrigin.cpp
[21/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/Context.cpp
[22/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/CanonicalIncludes.cpp
[23/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/AST.cpp
[24/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/FindTarget.cpp
[25/25] Processing file /Users/efritz/dev/sourcegraph/lsif-clang/index/Relation.cpp