[ARM][Driver][Windows] Allow command-line upgrade to Armv8.

If you gave clang the options `--target=arm-pc-windows-msvc` and
`-march=armv8-a+crypto` together, the crypto extension would not be
enabled in the compilation, and you'd see the following warning
message suggesting that the 'armv8-a' had been ignored:

  clang: warning: ignoring extension 'crypto' because the 'armv7-a' architecture does not support it [-Winvalid-command-line-argument]

This happens because Triple::getARMCPUForArch(), for the Win32 OS,
unconditionally returns "cortex-a9" (an Armv7 CPU) regardless of
MArch, which overrides the architecture setting on the command line.

I don't think that the combination of Windows and AArch32 _should_
unconditionally outlaw the use of the crypto extension. MSVC itself
doesn't think so: you can perfectly well compile Thumb crypto code
using its AArch32-targeted compiler.

All the other default CPUs in the same switch statement are
conditional on a particular MArch setting; this is the only one that
returns a particular CPU _regardless_ of MArch. So I've fixed this one
by adding a condition, so that if you ask for an architecture *above*
v7, the default of Cortex-A9 no longer overrides it.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D100937

lib/Support/Triple.cpp

@@ -1716,7 +1716,9 @@ StringRef Triple::getARMCPUForArch(StringRef MArch) const {
     break;
   case llvm::Triple::Win32:
     // FIXME: this is invalid for WindowsCE
-    return "cortex-a9";
+    if (ARM::parseArchVersion(MArch) <= 7)
+      return "cortex-a9";
+    break;
   case llvm::Triple::IOS:
   case llvm::Triple::MacOSX:
   case llvm::Triple::TvOS:

unittests/ADT/TripleTest.cpp

@@ -1511,6 +1511,7 @@ TEST(TripleTest, getARMCPUForArch) {
   {
     llvm::Triple Triple("arm--win32");
     EXPECT_EQ("cortex-a9", Triple.getARMCPUForArch());
+    EXPECT_EQ("generic", Triple.getARMCPUForArch("armv8-a"));
   }
   // Some alternative architectures
   {