[clangd] Add std::size_t to StdSymbol mapping

This is a common symbol that's missing from our mapping because
cppreference yields multiple headers.

Add it manually by picking cstddef to prevent insertion of some stdlib-internal
headers instead.

Fixes clangd/clangd#666.

Differential Revision: https://reviews.llvm.org/D95423

Author: kadircet

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

@@ -92,13 +92,17 @@ void CanonicalIncludes::addSystemHeadersMapping(const LangOptions &Language) {
 #include "StdSymbolMap.inc"
         // There are two std::move()s, this is by far the most common.
         SYMBOL(move, std::, <utility>)
+        // There are multiple headers for size_t, pick one.
+        SYMBOL(size_t, std::, <cstddef>)
 #undef SYMBOL
     });
     StdSymbolMapping = Symbols;
   } else if (Language.C11) {
     static const auto *CSymbols = new llvm::StringMap<llvm::StringRef>({
 #define SYMBOL(Name, NameSpace, Header) {#Name, #Header},
 #include "CSymbolMap.inc"
+        // There are multiple headers for size_t, pick one.
+        SYMBOL(size_t, None, <stddef.h>)
 #undef SYMBOL
     });
     StdSymbolMapping = CSymbols;