[RISCV][BF16] Enable __bf16 for riscv targets

The RISC-V psABI recently added __bf16 in riscv-non-isa/riscv-elf-psabi-doc#367.
Now we can enable this new type in clang.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D150929

clang/docs/LanguageExtensions.rst

@@ -816,6 +816,7 @@ to ``float``; see below for more information on this emulation.
 * ``__bf16`` is supported on the following targets (currently never natively):
   * 32-bit ARM
   * 64-bit ARM (AArch64)
+  * RISC-V
   * X86 (when SSE2 is available)
 
 (For X86, SSE2 is available on 64-bit and all recent 32-bit processors.)

clang/lib/Basic/Targets/RISCV.h

@@ -32,6 +32,9 @@ class RISCVTargetInfo : public TargetInfo {
 public:
   RISCVTargetInfo(const llvm::Triple &Triple, const TargetOptions &)
       : TargetInfo(Triple) {
+    BFloat16Width = 16;
+    BFloat16Align = 16;
+    BFloat16Format = &llvm::APFloat::BFloat();
     LongDoubleWidth = 128;
     LongDoubleAlign = 128;
     LongDoubleFormat = &llvm::APFloat::IEEEquad();
@@ -101,6 +104,8 @@ class RISCVTargetInfo : public TargetInfo {
 
   bool hasBitIntType() const override { return true; }
 
+  bool hasBFloat16Type() const override { return true; }
+
   bool useFP16ConversionIntrinsics() const override {
     return false;
   }