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

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.

Differential Revision: https://reviews.llvm.org/D88144

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

@@ -772,7 +772,10 @@ void CanonicalIncludes::addSystemHeadersMapping(const LangOptions &Language) {
                   MaxSuffixComponents;
          }) != SystemHeaderMap->keys().end());
 
-  StdSuffixHeaderMapping = SystemHeaderMap;
+  // FIXME: Suffix mapping contains invalid entries for C, so only enable it for
+  // CPP.
+  if (Language.CPlusPlus)
+    StdSuffixHeaderMapping = SystemHeaderMap;
 }
 
 } // namespace clangd

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

@@ -21,6 +21,10 @@ TEST(CanonicalIncludesTest, CStandardLibrary) {
   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) {