[Clang] Restore replace_path_prefix instead of startswith

In D49466, sys::path::replace_path_prefix was used instead startswith for -f[macro/debug/file]-prefix-map options.
However those were revert

[Clang] Restore replace_path_prefix instead of startswith

In D49466, sys::path::replace_path_prefix was used instead startswith for -f[macro/debug/file]-prefix-map options.
However those were reverted later (commit rG3bb24bf25767ef5bbcef958b484e7a06d8689204) due to broken Windows tests.

This patch restores those replace_path_prefix calls.
It also modifies the prefix matching to be case-insensitive under Windows.

Differential Revision : https://reviews.llvm.org/D76869

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