Preprocessor: Suppress -Wnonportable-include-path for header maps

If a file search involves a header map, suppress
-Wnonportable-include-path.  It's firing lots of false positives for
framework authors internally, and it's not trivial to fix.

Consider a framework called "Foo" with a main (installed) framework header
"Foo/Foo.h".  It's atypical for "Foo.h" to actually live inside a
directory called "Foo" in the source repository.  Instead, the
build system generates a header map while building the framework.
If Foo.h lives at the top-level of the source repository (common), and
the git repo is called ssh://some.url/foo.git, then the header map will
have something like:

    Foo/Foo.h -> /Users/myname/code/foo/Foo.h

where "/Users/myname/code/foo" is the clone of ssh://some.url/foo.git.

After #import <Foo/Foo.h>, the current implementation of
-Wnonportable-include-path will falsely assume that Foo.h was found in a
nonportable way, because of the name of the git clone (.../foo/Foo.h).
However, that directory name was not involved in the header search at
all.

This commit adds an extra parameter to Preprocessor::LookupFile and
HeaderSearch::LookupFile to track if the search used a header map,
making it easy to suppress the warning.  Longer term, once we find a way
to avoid the false positive, we should turn the warning back on.

rdar://problem/28863903

llvm-svn: 301592
diff --git a/clang/lib/Frontend/Rewrite/InclusionRewriter.cpp b/clang/lib/Frontend/Rewrite/InclusionRewriter.cpp
index d953da2..ca14da3 100644
--- a/clang/lib/Frontend/Rewrite/InclusionRewriter.cpp
+++ b/clang/lib/Frontend/Rewrite/InclusionRewriter.cpp
@@ -392,7 +392,7 @@
   // FIXME: Why don't we call PP.LookupFile here?
   const FileEntry *File = PP.getHeaderSearchInfo().LookupFile(
       Filename, SourceLocation(), isAngled, nullptr, CurDir, Includers, nullptr,
-      nullptr, nullptr, nullptr, false);
+      nullptr, nullptr, nullptr, nullptr);
 
   FileExists = File != nullptr;
   return true;