docs: standardize C/C++ language names to uppercase in documentation and code comments

[Copilot]

This PR standardizes all references to the C and C++ programming languages by converting lowercase instances to the proper uppercase format throughout the project documentation and code comments.

Changes Made

Documentation Updates:

• README.md: Changed "c/c++ lib" to "C/C++ lib" in the usage section

Code Comment Updates:

• _xtool/internal/clang/clang.go: Updated comment from "c/c++ standard" to "C/C++ standard" and "c++:gnu++17" to "C++:gnu++17"
• _xtool/internal/parser/parser.go: Changed "non cpp mode" to "non C++ mode" in developer note
• cmd/llcppcfg/llcppcfg.go: Updated flag description from "c++ lib" to "C++ lib"
• parser/iparser.go: Changed comment from "parser with c++" to "parser with C++"

What Was Not Changed

The changes were surgical and focused only on language name references. Technical terms, command names, file extensions, and JSON field names were preserved:

• Command line flags like -cpp and clang++
• File extensions like .cpp
• JSON field names like "c++": in symbol tables
• Code examples showing actual C/C++ syntax

All tests continue to pass and the build remains successful after these changes.

Fixes #536.

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[luoliwoshang]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.20%. Comparing base (30c4497) to head (83b6836).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #537   +/-   ##
=======================================
  Coverage   84.20%   84.20%           
=======================================
  Files          27       27           
  Lines        2747     2747           
=======================================
  Hits         2313     2313           
  Misses        391      391           
  Partials       43       43           

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[luoliwoshang]

@MeteorsLiu CC