Remove usage of <regex> from Path.

There was actually no need to give special treatment to leading drive
designators since they effectively form the first path segment anyway.
To help prevent regressions, I added a few unit tests in a previous CL.

The motivation for the CL is to remove a dependency on `locale.cpp`,
which can result in shorter build times and reduced binary sizes.

libs/utils/src/Path.cpp

@@ -19,7 +19,6 @@
 #include <sstream>
 #include <ostream>
 #include <iterator>
-#include <regex>
 
 #if defined(WIN32)
 #   include <utils/compiler.h>
@@ -185,12 +184,12 @@ std::vector<std::string> Path::split() const {
     size_t current;
     ssize_t next = -1;
 
-    // Matches a leading disk designator (C:\), forward slash (/), or back slash (\)
-    const static std::regex driveDesignationRegex(R"_regex(^([a-zA-Z]:\\|\\|\/))_regex");
-    std::smatch match;
-    if (std::regex_search(m_path, match, driveDesignationRegex)) {
-        segments.push_back(match[0]);
-        next = match[0].length() - 1;
+    // If there is a leading disk designator such as C:, this naturally becomes the first segment.
+    // However if there there is leading slash or back slash, we need to explicitly preserve it.
+    // Note that we are guaranteed to have at least one char in the path at this point.
+    if (m_path[0] == '/' || m_path[0] == '\\') {
+        segments.push_back(m_path.substr(0, 1));
+        next = 0;
     }
 
     do {