Build googletest with IntelLLVM compilers.

Use the same flags as Clang if the compiler id is IntelLLVM.

IntelLLVM warns if a double constant is assigned to a float.

```
[build] .../googletest/googletest/include/gtest/gtest-printers.h:516:17: warning: implicit conversion between floating point types of different sizes [-Wimplicit-float-size-conversion]
[build]       mulfor6 = 1e1;
```

IntelLLVM uses fp-model=fast by default, breaking IsNan and IsInf tests.
Use -ffp-model=precise to fix this.

IntelLLVM does not support -Wchar-subscripts

Fixes #4255

googletest/cmake/internal_utils.cmake

@@ -94,12 +94,18 @@ macro(config_compiler_and_linker)
     if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
       set(cxx_base_flags "${cxx_base_flags} -utf-8")
     endif()
-  elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" OR
+      CMAKE_CXX_COMPILER_ID STREQUAL "IntelLLVM")
     set(cxx_base_flags "-Wall -Wshadow -Wconversion -Wundef")
     set(cxx_exception_flags "-fexceptions")
     set(cxx_no_exception_flags "-fno-exceptions")
-    set(cxx_strict_flags "-W -Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch -Wunused-parameter -Wcast-align -Wchar-subscripts -Winline -Wredundant-decls")
+    set(cxx_strict_flags "-W -Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch -Wunused-parameter -Wcast-align -Winline -Wredundant-decls")
     set(cxx_no_rtti_flags "-fno-rtti")
+    if (CMAKE_CXX_COMPILER_ID STREQUAL "IntelLLVM")
+      set(cxx_base_flags "${cxx_base_flags} -Wno-implicit-float-size-conversion -ffp-model=precise")
+    else()
+      set(cxx_strict_flags "${cxx_strict_flags} -Wchar-subscripts")
+    endif()
   elseif (CMAKE_COMPILER_IS_GNUCXX)
     set(cxx_base_flags "-Wall -Wshadow -Wundef")
     if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.0.0)