[lldb] Fix guessing of windows path style

[jarin]

Summary

Currently, only paths with length>3 are guessed as Windows paths. This prevents root paths (such as C:\) to be recognized as Windows paths. This patch makes sure we also recognize "<letter>:\" as Windows paths.

lldb/source/Utility/FileSpec.cpp

bool FileSpec::Match(const FileSpec &pattern, const FileSpec &file) {
   return true;
 }
 
 llvm::Optional<FileSpec::Style> FileSpec::GuessPathStyle(llvm::StringRef absolute_path) {
   if (absolute_path.startswith("/"))
     return Style::posix;
   if (absolute_path.startswith(R"(\\)"))
     return Style::windows;
-  if (absolute_path.size() > 3 && llvm::isAlpha(absolute_path[0]) &&
+  if (absolute_path.size() >= 3 && llvm::isAlpha(absolute_path[0]) &&
       absolute_path.substr(1, 2) == R"(:\)")
     return Style::windows;
   return llvm::None;
 }
 
 // Dump the object to the supplied stream. If the object contains a valid
 // directory name, it will be displayed followed by a directory delimiter, and
 // the filename.

lldb/unittests/Utility/FileSpecTest.cpp

 
 TEST(FileSpecTest, GuessPathStyle) {
   EXPECT_EQ(FileSpec::Style::posix, FileSpec::GuessPathStyle("/foo/bar.txt"));
   EXPECT_EQ(FileSpec::Style::posix, FileSpec::GuessPathStyle("//net/bar.txt"));
   EXPECT_EQ(FileSpec::Style::windows,
             FileSpec::GuessPathStyle(R"(C:\foo.txt)"));
   EXPECT_EQ(FileSpec::Style::windows,
             FileSpec::GuessPathStyle(R"(\\net\foo.txt)"));
+  EXPECT_EQ(FileSpec::Style::windows, FileSpec::GuessPathStyle(R"(Z:\)"));
   EXPECT_EQ(llvm::None, FileSpec::GuessPathStyle("foo.txt"));
   EXPECT_EQ(llvm::None, FileSpec::GuessPathStyle("foo/bar.txt"));
+  EXPECT_EQ(llvm::None, FileSpec::GuessPathStyle("Z:"));
 }
 
 TEST(FileSpecTest, GetPath) {
   std::pair<const char *, const char *> posix_tests[] = {
       {"/foo/.././bar", "/bar"},
       {"/foo/./../bar", "/bar"},
       {"/foo/../bar", "/bar"},
       {"/foo/./bar", "/foo/bar"},