[Driver] Make the findVCToolChainViaEnvironment case-insensitive

[ArturGainullin]

Summary

PATH usage on Windows is case-insensitive. There could be situations
when toolchain path can't be obtained from PATH because of
case-sensitivity of the findVCToolChainViaEnvironment.

[rnk]

lgtm

This code could greatly benefit from some unit tests similar in style to the GCC installation detection tests, but It would be unreasonable for me to insist on them.

[rnk]

Example: https://github.com/llvm/llvm-project/blob/main/clang/unittests/Driver/ToolChainTest.cpp#L29

[ArturGainullin]

In D100361#2686823, @rnk wrote:

Example: https://github.com/llvm/llvm-project/blob/main/clang/unittests/Driver/ToolChainTest.cpp#L29

Thank you for the example! I will take a look and may consider adding a test as a follow up commit.

clang/lib/Driver/ToolChains/MSVC.cpp

for (llvm::StringRef PathEntry : PathEntries) {
       if (!IsBin) {
         // Strip any architecture subdir like "amd64".
         TestPath = llvm::sys::path::parent_path(TestPath);
         IsBin = llvm::sys::path::filename(TestPath).equals_lower("bin");
       }
       if (IsBin) {
         llvm::StringRef ParentPath = llvm::sys::path::parent_path(TestPath);
         llvm::StringRef ParentFilename = llvm::sys::path::filename(ParentPath);
-        if (ParentFilename == "VC") {
+        if (ParentFilename.equals_lower("VC")) {
           Path = std::string(ParentPath);
           VSLayout = MSVCToolChain::ToolsetLayout::OlderVS;
           return true;
         }
-        if (ParentFilename == "x86ret" || ParentFilename == "x86chk"
-          || ParentFilename == "amd64ret" || ParentFilename == "amd64chk") {
+        if (ParentFilename.equals_lower("x86ret") ||
+            ParentFilename.equals_lower("x86chk") ||
+            ParentFilename.equals_lower("amd64ret") ||
+            ParentFilename.equals_lower("amd64chk")) {
           Path = std::string(ParentPath);
           VSLayout = MSVCToolChain::ToolsetLayout::DevDivInternal;
           return true;
         }
 
       } else {
         // This could be a new (>=VS2017) toolchain. If it is, we should find
         // path components with these prefixes when walking backwards through
         // the path.
         // Note: empty strings match anything.
         llvm::StringRef ExpectedPrefixes[] = {"",     "Host",  "bin", "",
                                               "MSVC", "Tools", "VC"};
 
         auto It = llvm::sys::path::rbegin(PathEntry);
         auto End = llvm::sys::path::rend(PathEntry);
         for (llvm::StringRef Prefix : ExpectedPrefixes) {
           if (It == End)
             goto NotAToolChain;
-          if (!It->startswith(Prefix))
+          if (!It->startswith_lower(Prefix))
             goto NotAToolChain;
           ++It;
         }
 
         // We've found a new toolchain!
         // Back up 3 times (/bin/Host/arch) to get the root path.
         llvm::StringRef ToolChainPath(PathEntry);
         for (int i = 0; i < 3; ++i)