[IR] Make DataLayout compute canonical data layouts

Clang and other frontends generally need the LLVM data layout string in
order to generate LLVM IR modules for LLVM. MLIR clients often need it
as well, since MLIR users often lower to LLVM IR.

Before this change, the LLVM datalayout string was computed in the
LLVM${TGT}CodeGen library in the relevant TargetMachine subclass.
However, none of the logic for computing the data layout string requires
any details of code generation. Clients who want to avoid duplicating
this information were forced to link in LLVMCodeGen and all registered
targets, leading to bloated binaries.

By moving this information to the DataLayout class in the IR file, we
can delete the duplicate datalayout strings in Clang, and retain the
ability to generate IR for unregistered targets.

Author: rnk

llvm/include/llvm/IR/DataLayout.h

@@ -194,6 +194,12 @@ class DataLayout {
   /// description on failure.
   LLVM_ABI static Expected<DataLayout> parse(StringRef LayoutString);
 
+  /// Given an LLVM target triple, compute the well-known datalayout string.
+  /// TODO: Create a static factor helper version of this method and refactor
+  /// the TargetMachine callers to use it.
+  LLVM_ABI static std::string computeStringForTriple(const Triple &T,
+                                                     StringRef ABIName = "");
+
   /// Layout endianness...
   bool isLittleEndian() const { return !BigEndian; }
   bool isBigEndian() const { return BigEndian; }