[ADT] Remove APInt/APSInt toString() std::string variants

<string> is currently the highest impact header in a clang+llvm build:

https://commondatastorage.googleapis.com/chromium-browser-clang/llvm-include-analysis.html

One of the most common places this is being included is the APInt.h header, which needs it for an old toString() implementation that returns std::string - an inefficient method compared to the SmallString versions that it actually wraps.

This patch replaces these APInt/APSInt methods with a pair of llvm::toString() helpers inside StringExtras.h, adjusts users accordingly and removes the <string> from APInt.h - I was hoping that more of these users could be converted to use the SmallString methods, but it appears that most end up creating a std::string anyhow. I avoided trying to use the raw_ostream << operators as well as I didn't want to lose having the integer radix explicit in the code.

Differential Revision: https://reviews.llvm.org/D103888

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -1930,7 +1930,7 @@ const MCExpr *TargetLoweringObjectFileCOFF::lowerRelativeReference(
 
 static std::string APIntToHexString(const APInt &AI) {
   unsigned Width = (AI.getBitWidth() / 8) * 2;
-  std::string HexString = AI.toString(16, /*Signed=*/false);
+  std::string HexString = toString(AI, 16, /*Signed=*/false);
   llvm::transform(HexString, HexString.begin(), tolower);
   unsigned Size = HexString.size();
   assert(Width >= Size && "hex string is too large!");