[clangd] Disable suffix matching fallback for C during include insertion

[kadircet]

Summary

Clangd currently doesn't respect language and breaks the builds with
include insertion for C. This patch aims to stop the bleeding by not mapping
back to CPP standard library headers.

Improves https://github.com/clangd/clangd/issues/376.

[sammccall]

(I do wonder whether it's safe to just drop the mapping table entirely now...)

[kadircet]

In D88144#2290830, @sammccall wrote:

(I do wonder whether it's safe to just drop the mapping table entirely now...)

As discussed offline;
It still helps with mapping symbols missing from our cppreference extraction back to umbrella headers. Not sure how often this happens in practice anymore, but it will likely be a regression.
So gonna drop it completely in a different patch, which can be reverted quickly if things go wrong. That way we'll learn about what's currently missing, rather than masking the bugs.

clang-tools-extra/clangd/index/CanonicalIncludes.cpp

#undef SYMBOL
   // ... and precise.
   assert(llvm::find_if(SystemHeaderMap->keys(), [](llvm::StringRef Path) {
            return std::distance(llvm::sys::path::begin(
                                     Path, llvm::sys::path::Style::posix),
                                 llvm::sys::path::end(Path)) ==
                   MaxSuffixComponents;
          }) != SystemHeaderMap->keys().end());
 
+  // FIXME: Suffix mapping contains invalid entries for C, so only enable it for
+  // CPP.
+  if (Language.CPlusPlus)
     StdSuffixHeaderMapping = SystemHeaderMap;
 }
 
 } // namespace clangd
 } // namespace clang

clang-tools-extra/clangd/unittests/CanonicalIncludesTests.cpp

 
 TEST(CanonicalIncludesTest, CStandardLibrary) {
   CanonicalIncludes CI;
   auto Language = LangOptions();
   Language.C11 = true;
   CI.addSystemHeadersMapping(Language);
   // Usual standard library symbols are mapped correctly.
   EXPECT_EQ("<stdio.h>", CI.mapHeader("path/stdio.h", "printf"));
+  // Suffix mapping isn't available for C, instead of mapping to `<cstdio> we
+  // just leave the header as-is.
+  EXPECT_EQ("include/stdio.h",
+            CI.mapHeader("include/stdio.h", "unknown_symbol"));
 }
 
 TEST(CanonicalIncludesTest, CXXStandardLibrary) {
   CanonicalIncludes CI;
   auto Language = LangOptions();
   Language.CPlusPlus = true;
   CI.addSystemHeadersMapping(Language);