[OpenMP] Prevent nesting of target constructs within target code execution regions.

Summary:
This patch enhances Sema to check for the following restriction:

OpenMP 4.5 [2.17 Nesting of Regions]
If a target, target update, target data, target enter data, or
target exit data construct is encountered during execution of a
target region, the behavior is unspecified.

Reviewers: ABataev

Differential Revision: http://reviews.llvm.org/D16758

llvm-svn: 259464
diff --git a/clang/test/OpenMP/target_parallel_device_messages.cpp b/clang/test/OpenMP/target_parallel_device_messages.cpp
index 7c03975..6c8d4c2 100644
--- a/clang/test/OpenMP/target_parallel_device_messages.cpp
+++ b/clang/test/OpenMP/target_parallel_device_messages.cpp
@@ -11,16 +11,27 @@
 
 int main(int argc, char **argv) {
   #pragma omp target parallel device // expected-error {{expected '(' after 'device'}}
+  foo();
   #pragma omp target parallel device ( // expected-error {{expected expression}} expected-error {{expected ')'}} expected-note {{to match this '('}}
+  foo();
   #pragma omp target parallel device () // expected-error {{expected expression}}
+  foo();
   #pragma omp target parallel device (argc // expected-error {{expected ')'}} expected-note {{to match this '('}}
+  foo();
   #pragma omp target parallel device (argc)) // expected-warning {{extra tokens at the end of '#pragma omp target parallel' are ignored}}
-#pragma omp target parallel device (argc > 0 ? argv[1] : argv[2]) // expected-error {{expression must have integral or unscoped enumeration type, not 'char *'}}
+  foo();
+  #pragma omp target parallel device (argc > 0 ? argv[1] : argv[2]) // expected-error {{expression must have integral or unscoped enumeration type, not 'char *'}}
+  foo();
   #pragma omp target parallel device (argc + argc)
+  foo();
   #pragma omp target parallel device (argc), device (argc+1) // expected-error {{directive '#pragma omp target parallel' cannot contain more than one 'device' clause}}
+  foo();
   #pragma omp target parallel device (S1) // expected-error {{'S1' does not refer to a value}}
+  foo();
   #pragma omp target parallel device (-2) // expected-error {{argument to 'device' clause must be a non-negative integer value}}
+  foo();
   #pragma omp target parallel device (-10u)
+  foo();
   #pragma omp target parallel device (3.14) // expected-error {{expression must have integral or unscoped enumeration type, not 'double'}}
   foo();