clang: Diag running out of file handles while looking for files
clang: Diag running out of file handles while looking for files

clang would only print "file not found" when it's unable to find a
header file.  If the reason for that is a file handle leak, that's not a
very useful error message.  For errors that aren't in a small whitelist
("file not found", "file is directory"), print an error with the
strerror() output.

This changes behavior in corner cases: If clang was out of file handles
while looking in one -I dir but then suddenly wasn't when looking in the
next -I dir, and both directories contained a file with the desired
name, previously we'd silently return the file from the second
directory. For this reason, it's important to ignore "is a directory"
for this new diag: if a file foo/foo exists and -I -Ifoo are passed, an
include of "foo" should successfully open file "foo" in directory "foo/"
instead of complaining that "./foo" is a directory.

No test since we mostly hit this when there's a handle leak somewhere,
and currently there isn't one. I manually tested this with the repro
steps in comment 2 on the bug below.

Fixes PR42524.

Differential Revision: https://reviews.llvm.org/D65956

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@368322 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 442c97abef7f1986a67588c9ad59af875918b53a)

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