[clangd] Add std::size_t to StdSymbol mapping

[kadircet]

Summary

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 https://github.com/clangd/clangd/issues/666.

[sammccall]

This might annoy people occasionally, but so does the current behavior.
I guess inserting *nothing* might be better, we could have a list of symbols known to have no insertable header. But we don't have that today, and this is an improvement we can land before the 12 release.

stddef seems like the best choice among the headers that provide size_t.

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

 
 void CanonicalIncludes::addSystemHeadersMapping(const LangOptions &Language) {
   if (Language.CPlusPlus) {
     static const auto *Symbols = new llvm::StringMap<llvm::StringRef>({
 #define SYMBOL(Name, NameSpace, Header) {#NameSpace #Name, #Header},
 #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;
   }
 
   // FIXME: remove the std header mapping once we support ambiguous symbols, now
   // it serves as a fallback to disambiguate:
   //   - symbols with multiple headers (e.g. std::move)