fix python SyntaxWarnings (invalid escape sequence) in check-all output

[eleftg]

Such as:

src_dir/clang/lib/Tooling/DumpTool/generate_cxx_src_locs.py:28: SyntaxWarning: invalid escape sequence '\*'    self.implementationContent += """
src_dir/llvm/utils/lit/lit/TestRunner.py:205: SyntaxWarning: invalid escape sequence '\c'                      """
src_dir/llvm/utils/lit/lit/TestRunner.py:1561: SyntaxWarning: invalid escape sequence '\s'                     match = _caching_re_compile("^\s*%else\s*(%{)?").search(ln)
src_dir/libcxx/utils/libcxx/test/format.py:64: SyntaxWarning: invalid escape sequence '\s'                     for output in re.split('[$]\s*":"\s*"RUN: at line \d+"', fullOutput):
src_dir/libcxx/utils/libcxx/test/params.py:121: SyntaxWarning: invalid escape sequence '\+'                    AddSubstitution("%{cxx_std}", re.sub("\+", "x", std)),
src_dir/libcxx/utils/libcxx/test/params.py:214: SyntaxWarning: invalid escape sequence '\+'                    AddFeature("stdlib=libc++") if re.match(".+-libc\+\+", stdlib) else None,
src_dir/compiler-rt/test/lit.common.cfg.py:800: SyntaxWarning: invalid escape sequence '\$'                    "-Wl,-z,origin -Wl,-rpath,\$ORIGIN -L%T -l%xdynamiclib_namespec"
src_dir/compiler-rt/test/lit.common.cfg.py:809: SyntaxWarning: invalid escape sequence '\$'                    "-Wl,-rpath,\$ORIGIN -L%T -l%xdynamiclib_namespec" + postfix,
src_dir/compiler-rt/test/lit.common.cfg.py:817: SyntaxWarning: invalid escape sequence '\$'                    "-Wl,-R\$ORIGIN -L%T -l%xdynamiclib_namespec" + postfix,
src_dir/llvm/test/lit.cfg.py:275: SyntaxWarning: invalid escape sequence '\d'                                  match = re.search("release (\d+)\.(\d+)", ptxas_out)

Converting to raw strings addresses the issue.

Thanks a lot @ldionne for pointing me to the right direction, on properly re-submitting this pull request.

cc: @petrhosek

/branch eleftg/llvm-project/python_SyntaxWarnings_backport_17

[eleftg]

/branch eleftg/llvm-project/python_SyntaxWarnings_backport_17

[llvmbot]

/branch eleftg/llvm-project/python_SyntaxWarnings_backport_17

Error: Command failed due to missing milestone.

[eleftg]

@ldionne Unfortunately, despite using this to create the issue, it looks like the Milestone is silently dropped right after creation.

[EugeneZelenko]

/branch eleftg/llvm-project/python_SyntaxWarnings_backport_17

[llvmbot]

/pull-request llvm/llvm-project-release-prs#782