Rename top-level LICENSE.txt files to LICENSE.TXT

This makes all the license filenames uniform across subprojects.

Differential Revision: https://reviews.llvm.org/D98380

diff --git a/flang/LICENSE.txt b/flang/LICENSE.TXT
similarity index 100%
rename from flang/LICENSE.txt
rename to flang/LICENSE.TXT

diff --git a/libc/LICENSE.txt b/libc/LICENSE.TXT
similarity index 100%
rename from libc/LICENSE.txt
rename to libc/LICENSE.TXT

diff --git a/openmp/LICENSE.txt b/openmp/LICENSE.TXT
similarity index 100%
rename from openmp/LICENSE.txt
rename to openmp/LICENSE.TXT

diff --git a/polly/LICENSE.txt b/polly/LICENSE.TXT
similarity index 100%
rename from polly/LICENSE.txt
rename to polly/LICENSE.TXT

diff --git a/pstl/LICENSE.txt b/pstl/LICENSE.TXT
similarity index 100%
rename from pstl/LICENSE.txt
rename to pstl/LICENSE.TXT