Apply modernize-use-starts-ends-with on llvm-project (#89140)

Run `modernize-use-starts-ends-with` on llvm-project. Two instances are
flagged, minor readability improvements, extremely minor performance
improvements.

```
python3 clang-tools-extra/clang-tidy/tool/run-clang-tidy.py \
    -clang-tidy-binary="build/bin/clang-tidy" \
    -clang-apply-replacements-binary="build/bin/clang-apply-replacements" \
    -checks="-*,modernize-use-starts-ends-with" \
    -header-filter=".*" \
    -fix -format
```

I am working on some additions to this check, but they don't seem to
flag any additional cases anyway.

clang-tools-extra/clang-doc/Representation.cpp

@@ -380,8 +380,8 @@ ClangDocContext::ClangDocContext(tooling::ExecutionContext *ECtx,
   this->SourceRoot = std::string(SourceRootDir);
   if (!RepositoryUrl.empty()) {
     this->RepositoryUrl = std::string(RepositoryUrl);
-    if (!RepositoryUrl.empty() && RepositoryUrl.find("http://") != 0 &&
-        RepositoryUrl.find("https://") != 0)
+    if (!RepositoryUrl.empty() && !RepositoryUrl.starts_with("http://") &&
+        !RepositoryUrl.starts_with("https://"))
       this->RepositoryUrl->insert(0, "https://");
   }
 }

lldb/unittests/Host/FileSystemTest.cpp

@@ -93,7 +93,7 @@ class DummyFileSystem : public vfs::FileSystem {
     std::map<std::string, vfs::Status>::iterator I;
     std::string Path;
     bool isInPath(StringRef S) {
-      if (Path.size() < S.size() && S.find(Path) == 0) {
+      if (Path.size() < S.size() && S.starts_with(Path)) {
         auto LastSep = S.find_last_of('/');
         if (LastSep == Path.size() || LastSep == Path.size() - 1)
           return true;

llvm/unittests/Support/VirtualFileSystemTest.cpp

@@ -101,7 +101,7 @@ class DummyFileSystem : public vfs::FileSystem {
     std::map<std::string, vfs::Status>::iterator I;
     std::string Path;
     bool isInPath(StringRef S) {
-      if (Path.size() < S.size() && S.find(Path) == 0) {
+      if (Path.size() < S.size() && S.starts_with(Path)) {
         auto LastSep = S.find_last_of('/');
         if (LastSep == Path.size() || LastSep == Path.size() - 1)
           return true;