[OpenMP] Fix runtimes default build (#149871)

The default build of openmp (`cmake -S <llvm-project>/runtimes
-DLLVM_ENABLE_RUNTIMES=openmp`) current fails with
```
CMake Error at /home/meinersbur/src/llvm/flangrt/_src/cmake/Modules/GetClangResourceDir.cmake:17 (string):
  string sub-command REGEX, mode MATCH needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  /home/meinersbur/src/llvm/flangrt/_src/openmp/CMakeLists.txt:126 (get_clang_resource_dir)
```
The reason is that because it is not a bootstrapping-build, the clang
resource dir that it intends to write files such as `omp-tools.h` into,
is unavailable. Using the Clang resource dir for writing files is
conceptually broken, as that dir might be located in
`/usr/lib/clang/<version>/`. Writing to it is only intended in
bootstrapping builds where Clang is built alongside openmp.

This patch unifies the identification of being in a bootstrapping built.
The same `LLVM_TREE_AVAILABLE` definition is going to be used in
#137828. No reason for each runtime to define its own.

1 files changed, 8 insertions, 0 deletions

diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index e4dd4ebfc678..d0d2a538e1e8 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -92,6 +92,14 @@ include(LLVMCheckCompilerLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
+
+# Determine whether we are in the runtimes/runtimes-bins directory of a
+# bootstrap build.
+set(LLVM_TREE_AVAILABLE OFF)
+if (LLVM_LIBRARY_OUTPUT_INTDIR AND LLVM_RUNTIME_OUTPUT_INTDIR AND PACKAGE_VERSION)
+  set(LLVM_TREE_AVAILABLE ON)
+endif()
+
 # CMake omits default compiler include paths, but in runtimes build, we use
 # -nostdinc and -nostdinc++ and control include paths manually so this behavior
 # is undesirable. Filtering CMAKE_{LANG}_IMPLICIT_INCLUDE_DIRECTORIES to remove